kbuild: lto: add _mcount to list of used symbols

Some randconfig builds fail with undefined references to _mcount
when CONFIG_TRIM_UNUSED_KSYMS is set:

ERROR: modpost: "_mcount" [drivers/tee/optee/optee.ko] undefined!
ERROR: modpost: "_mcount" [drivers/fsi/fsi-occ.ko] undefined!
ERROR: modpost: "_mcount" [drivers/fpga/dfl-pci.ko] undefined!

Since there is already a list of symbols that get generated at link
time, add this one as well.

Fixes: fbe078d397b4 ("kbuild: lto: add a default list of used symbols")
Signed-off-by: Arnd Bergmann <[e-mail]>
Signed-off-by: Masahiro Yamada <[e-mail]>

scripts/lto-used-symbollist.txt

···
 memcpy
 memmove
 memset
+_mcount
 __stack_chk_fail
 __stack_chk_guard