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

[Florian Weimer]

* Sunil Pandey:

> On Wed, Nov 1, 2023 at 11:44 PM Florian Weimer <fw@deneb.enyo.de> wrote:
>
>> * Noah Goldstein:
>>
>> > Okay bug is missing VBMI2 check. But the VBMI2 stuff
>> > isn't really needed so will update and repost w/ fixed ISA.
>>
>> Thanks.  The missing check explains why I couldn't reproduce it on the
>> lab machines I tried, which must have had VBMI2.  The Fedora builders
>> apparently don't.
>
> I would like to backport this patch to release branches.
> Any comments or objections?

You mean backporting the optimization *and* the bug fix?  No specific
concern from me.

However, we have an unresolved FreeBSD bhyve compatibility issue:

  glibc-2.39-2 and above on the host segfault 
  <https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=279901>

If we backport the x86-64 string function optimization, we will likely
end up with that problem on earlier releases, too.

Thanks,
Florian