x86_64: Add avx2 optimized __memcmpeq in memcmpeq-avx2.S

No bug. This commit adds new optimized __memcmpeq implementation for
avx2.

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.