The optimizer will now convert calls to memcmp into a calls to bcmp
in some circumstances. Users who are building freestanding code (not
depending on the platform’s libc) without specifying -ffreestanding may
need to either pass -fno-builtin-bcmp, or provide a bcmp function.

http://llvm.org/docs/ReleaseNotes.html#non-comprehensive-list-of-changes-in-this-release