amd64: make bcmp in libc just call memcmp

[mjg]

Summary

Preferably bcmp would just alias memcmp but there is magic which makes
this problematic.

A note should be made that bcmp which finds a difference has less work
to do but it's not worth implementing it just for this case.

This has a side effect of syncing assembly missed on memcmp rewrite.

[kib]

lib/libc/amd64/string/bcmp.c

3

The comment should be updated, same as for strlen().

[arichardson]

FYI as of LLVM 9, clang will optimize memcmp calls that only check for equality to bcmp.

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.

[mjg]

Huh, that changes things. In this case I may end up implementing a dedicated bcmp for both the kernel and libc.