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

[Florian Weimer]

* Noah Goldstein:

> 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.

I believe this caused some sort of regression because when we upgraded
glibc in the Fedora rawhide buildroot, a lot of things started failing:

  glibc-2.38.9000-13.fc40 broke rawhide buildroot on x86_64
  <https://bugzilla.redhat.com/show_bug.cgi?id=2244688>

The list of changes relative to the previous version is rather short:

- stdlib: fix grouping verification with multi-byte thousands separator (bug 30964)
- build-many-glibcs: Check for required system tools
- x86: Prepare `strrchr-evex` and `strrchr-evex512` for AVX10
- aarch64: Optimise vecmath logs
- aarch64: Cosmetic change in SVE exp routines
- aarch64: Optimize SVE cos & cosf
- aarch64: Improve vecmath sin routines
- nss: Get rid of alloca usage in makedb's write_output.
- debug: Add regression tests for BZ 30932
- Fix FORTIFY_SOURCE false positive
- nss: Rearrange and sort Makefile variables
- inet: Rearrange and sort Makefile variables
- Fix off-by-one OOB write in iconv/tst-iconv-mt

And this patch is the most likely one to cause issues.  I will try to
revert the patch and see if it fixes the observed issues.

Thanks,
Florian