[BOLT] Initial support for memcpy() inlininig

Summary:
Add "-inline-memcpy" option to inline calls to memcpy() using
"rep movsb" instruction. The pass is X86-specific.

Calls to _memcpy8 are optimized too using a special return value
(dest+size).

The implementation is very primitive in that it does not track liveness
of %rax after return, and no %rcx substitution. This is going to get
improved if we find the optimization to be useful.

(cherry picked from FBD8211890)