x86_64: Add evex optimized __memcmpeq in memcmpeq-evex.S

No bug. This commit adds new optimized __memcmpeq implementation for
evex.

The primary optimizations are:

1) skipping the logic to find the difference of the first mismatched
byte.

2) not updating src/dst addresses as the non-equals logic does not
need to be reused by different areas.