x86_64: Implement evex512 version of memchr, rawmemchr and wmemchr

This patch implements following evex512 version of string functions.
evex512 version takes up to 30% less cycle as compared to evex,
depending on length and alignment.

- memchr function using 512 bit vectors.
- rawmemchr function using 512 bit vectors.
- wmemchr function using 512 bit vectors.

Code size data:

memchr-evex.o		762 byte
memchr-evex512.o	576 byte (-24%)

rawmemchr-evex.o	461 byte
rawmemchr-evex512.o	412 byte (-11%)

wmemchr-evex.o		794 byte
wmemchr-evex512.o	552 byte (-30%)

Placeholder function, not used by any processor at the moment.

Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>