x86: Unifies 'strlen-evex' and 'strlen-evex512' implementations.

x86: Unifies 'strlen-evex' and 'strlen-evex512' implementations.

This commit uses a common implementation 'strlen-evex-base.S' for both
'strlen-evex' and 'strlen-evex512'

The motivation is to reduce the number of implementations to maintain.
This incidentally gives a small performance improvement.

All tests pass on x86.

Benchmarks were taken on SKX.
https://www.intel.com/content/www/us/en/products/sku/123613/intel-core-i97900x-xseries-processor-13-75m-cache-up-to-4-30-ghz/specifications.html

Geometric mean for strlen-evex512 over all benchmarks (N=10) was (new/old) 0.939
Geometric mean for wcslen-evex512 over all benchmarks (N=10) was (new/old) 0.965

Code Size Changes:
    strlen-evex512.S    :  +24 bytes
    wcslen-evex512.S    :  +54 bytes

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