diff -u linux-success/Module.symvers linux-fail/Module.symvers
-0x00000000     __clear_user    vmlinux EXPORT_SYMBOL
-0x00000000     clear_user      vmlinux EXPORT_SYMBOL

diff -u <(cut -d " " -f 2- ./linux-success/System.map) <(cut -d " " -f 2- ./linux-fail/System.map)
-r __ksymtab___clear_user
-r __ksymtab_clear_user
-r __kstrtab_clear_user
-r __kstrtab___clear_user

diff -u <(cut -d " " -f 2- ./linux-success/.tmp_System.map) <(cut -d " " -f 2- ./linux-fail/.tmp_System.map)
-r __ksymtab___clear_user
-r __ksymtab_clear_user
-r __kstrtab_clear_user
-r __kstrtab___clear_user

I had to cut away the first field from the system.map files because it always differ.
.tmp_kallsyms[12].S also differ a lot but I don't know what to make of those files.
There is no difference to any file in include/config/ksym or any *.cmd file except
some *.cmd files related to modules only exist in the success folder.

I patched the kernel with "kbuild: show the list of exported symbols with V=1 for TRIM_UNUSED_KSYMS"
and ran the build with V=1 until I got a failure. Here is the full output:

success: https://gist.githubusercontent.com/anonymous/86325ce634568f1e1d31c23b5b9df295/raw/a897c77f34499a990505057f0221b4a935db4a16/gistfile1.txt
fail: https://gist.githubusercontent.com/anonymous/279a7925f1a8cd05dcad78dca0720688/raw/9e407b262fdac284b15ae07f2d7a7824051d56cc/gistfile1.txt

The error was:
ERROR: "clear_user" [drivers/media/v4l2-core/videodev.ko] undefined!
ERROR: "__clear_user" [arch/x86/kvm/kvm.ko] undefined!

and the relevant part of the diff of the build:

--- kernel_build_debug_patch_success    2018-03-13 14:47:24.000000000 +0100
+++ kernel_build_debug_patch_fail       2018-03-13 15:12:33.000000000 +0100
@@ -45298,13 +45298,6 @@
 KSYM___x86_indirect_thunk_r15
 KSYM___fill_rsb
 KSYM___clear_rsb
-  gcc -Wp,-MD,arch/x86/lib/.usercopy_64.o.d  -nostdinc -isystem /usr/lib/gcc/x86_64-pc-linux-gnu/6.4.0/include -I./arch/x86/include -I./arch/x86/include/generated  -I./include -I./arch/x86/include/uapi -I./arch/x86/include/generated/uapi -I./include/uapi -I./include/generated/uapi -include ./include/linux/kconfig.h -D__KERNEL__ -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -fshort-wchar -Werror-implicit-function-declaration -Wno-format-security -std=gnu89 -fno-PIE -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx -m64 -falign-jumps=1 -falign-loops=1 -mno-80387 -mno-fp-ret-in-387 -mpreferred-stack-boundary=3 -mskip-rax-setup -march=core2 -mno-red-zone -mcmodel=kernel -funit-at-a-time -DCONFIG_AS_CFI=1 -DCONFIG_AS_CFI_SIGNAL_FRAME=1 -DCONFIG_AS_CFI_SECTIONS=1 -DCONFIG_AS_FXSAVEQ=1 -DCONFIG_AS_SSSE3=1 -DCONFIG_AS_CRC32=1 -DCONFIG_AS_AVX=1 -DCONFIG_AS_AVX2=1 -DCONFIG_AS_AVX512=1 -DCONFIG_AS_SHA1_NI=1 -DCONFIG_AS_SHA256_NI=1 -pipe -Wno-sign-compare -fno-asynchronous-unwind-tables -fno-delete-null-pointer-checks -Wno-frame-address -O2 --param=allow-store-data-races=0 -DCC_HAVE_ASM_GOTO -fplugin=./scripts/gcc-plugins/latent_entropy_plugin.so -fplugin=./scripts/gcc-plugins/structleak_plugin.so -DLATENT_ENTROPY_PLUGIN -fplugin-arg-structleak_plugin-byref-all -DSTRUCTLEAK_PLUGIN -Wframe-larger-than=1024 -fstack-protector-strong -Wno-unused-but-set-variable -Wno-unused-const-variable -fno-var-tracking-assignments -g -pg -mfentry -DCC_USING_FENTRY -fno-inline-functions-called-once -Wdeclaration-after-statement -Wno-pointer-sign -fno-strict-overflow -fno-stack-check -fconserve-stack -Werror=implicit-int -Werror=strict-prototypes -Werror=date-time -Werror=incompatible-pointer-types -Werror=designated-init    -DKBUILD_BASENAME='"usercopy_64"'  -DKBUILD_MODNAME='"usercopy_64"' -c -o arch/x86/lib/usercopy_64.o arch/x86/lib/usercopy_64.c
-KSYM___clear_user
-KSYM_clear_user
-KSYM_arch_wb_cache_pmem
-KSYM_memcpy_flushcache
-   ./tools/objtool/objtool orc generate --no-fp  "arch/x86/lib/usercopy_64.o";
-  if [ "-pg" = "-pg" ]; then if [ arch/x86/lib/usercopy_64.o != "scripts/mod/empty.o" ]; then ./scripts/recordmcount  "arch/x86/lib/usercopy_64.o"; fi; fi;
   rm -f arch/x86/lib/lib.a; ar rcsTPD arch/x86/lib/lib.a arch/x86/lib/clear_page_64.o arch/x86/lib/cmdline.o arch/x86/lib/cmpxchg16b_emu.o arch/x86/lib/copy_page_64.o arch/x86/lib/copy_user_64.o arch/x86/lib/cpu.o arch/x86/lib/csum-copy_64.o arch/x86/lib/csum-partial_64.o arch/x86/lib/csum-wrappers_64.o arch/x86/lib/delay.o arch/x86/lib/getuser.o arch/x86/lib/inat.o arch/x86/lib/insn.o arch/x86/lib/kaslr.o arch/x86/lib/memcpy_64.o arch/x86/lib/memmove_64.o arch/x86/lib/memset_64.o arch/x86/lib/misc.o arch/x86/lib/putuser.o arch/x86/lib/retpoline.o arch/x86/lib/rwsem.o arch/x86/lib/usercopy.o arch/x86/lib/usercopy_64.o
   objdump -h arch/x86/lib/lib.a | sed -ne '/___ksymtab/s/.*+\([^ ]*\).*/EXTERN(\1)/p' >arch/x86/lib/.lib-ksyms.o.lds; rm -f arch/x86/lib/.lib_exports.o; echo | gcc -Wp,-MD,arch/x86/lib/.lib-ksyms.o.d  -nostdinc -isystem /usr/lib/gcc/x86_64-pc-linux-gnu/6.4.0/include -I./arch/x86/include -I./arch/x86/include/generated  -I./include -I./arch/x86/include/uapi -I./arch/x86/include/generated/uapi -I./include/uapi -I./include/generated/uapi -include ./include/linux/kconfig.h -D__KERNEL__ -D__ASSEMBLY__ -fno-PIE -m64 -DCONFIG_AS_CFI=1 -DCONFIG_AS_CFI_SIGNAL_FRAME=1 -DCONFIG_AS_CFI_SECTIONS=1 -DCONFIG_AS_FXSAVEQ=1 -DCONFIG_AS_SSSE3=1 -DCONFIG_AS_CRC32=1 -DCONFIG_AS_AVX=1 -DCONFIG_AS_AVX2=1 -DCONFIG_AS_AVX512=1 -DCONFIG_AS_SHA1_NI=1 -DCONFIG_AS_SHA256_NI=1 -DCC_HAVE_ASM_GOTO -Wa,-gdwarf-2 -mfentry -DCC_USING_FENTRY   -c -o arch/x86/lib/.lib_exports.o -x assembler -; ld -m elf_x86_64   -r -o arch/x86/lib/lib-ksyms.o -T arch/x86/lib/.lib-ksyms.o.lds arch/x86/lib/.lib_exports.o; rm arch/x86/lib/.lib_exports.o arch/x86/lib/.lib-ksyms.o.lds
    rm -f arch/x86/lib/built-in.o; ar rcSTPD arch/x86/lib/built-in.o arch/x86/lib/msr-smp.o arch/x86/lib/cache-smp.o arch/x86/lib/msr.o arch/x86/lib/msr-reg.o arch/x86/lib/msr-reg-export.o arch/x86/lib/hweight.o arch/x86/lib/iomap_copy_64.o arch/x86/lib/lib-ksyms.o ; scripts/mod/modpost arch/x86/lib/built-in.o
