x86_64: Implement evex512 version of strchrnul, strchr and wcschr

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.

- strchrnul function using 512 bit vectors.
- strchr function using 512 bit vectors.
- wcschr function using 512 bit vectors.

Code size data:

strchrnul-evex.o	599 byte
strchrnul-evex512.o	569 byte (-5%)

strchr-evex.o		639 byte
strchr-evex512.o	595 byte (-7%)

wcschr-evex.o		644 byte
wcschr-evex512.o	607 byte (-6%)

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

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