x86: Prepare `strrchr-evex` and `strrchr-evex512` for AVX10

x86: Prepare `strrchr-evex` and `strrchr-evex512` for AVX10

This commit refactors `strrchr-evex` and `strrchr-evex512` to use a
common implementation: `strrchr-evex-base.S`.

The motivation is `strrchr-evex` needed to be refactored to not use
64-bit masked registers in preperation for AVX10.

Once vec-width masked register combining was removed, the EVEX and
EVEX512 implementations can easily be implemented in the same file
without any major overhead.

The net result is performance improvements (measured on TGL) for both
`strrchr-evex` and `strrchr-evex512`. Although, note there are some
regressions in the test suite and it may be many of the cases that
make the total-geomean of improvement/regression across bench-strrchr
are cold. The point of the performance measurement is to show there
are no major regressions, but the primary motivation is preperation
for AVX10.

Benchmarks where taken on TGL:
https://www.intel.com/content/www/us/en/products/sku/213799/intel-core-i711850h-processor-24m-cache-up-to-4-80-ghz/specifications.html

EVEX geometric_mean(N=5) of all benchmarks New / Original   : 0.74
EVEX512 geometric_mean(N=5) of all benchmarks New / Original: 0.87

Full check passes on x86.