feat(libc): implement memrchr

[MarcZX10]

What

Implement the byte-wise reverse search used by memrchr, including conversion of the search value to unsigned char. Add a dedicated guest test for repeated matches, bounded searches, high byte values, and misses.

Tested

Linux, GCC 12.2.0: guest_memrchr passed; conventions check and git diff --check passed.

Checklist

• Based on current main; no open PR implements memrchr
• One topic per PR; test is isolated from GuestStrings.cpp changes in test(libc): cover strlcpy and case-insensitive comparisons #1490
• No comments or investigation files added
• Generic POSIX behavior
• Depends on: none
• AI-assisted: yes

[oneandonlydean]

No title reaches memrchr on main yet: the homebrew audit lists the import, but those titles stop earlier at other missing imports. Closing for now; reopen when it's a title's next stop on main, with the log line.

[Amirtheshwaran]

@MarcZX100 heads-up: with #1848, #2090, #1772, #1650 and #1624 merged, PICO-8 for PS5 (PPSA99808) now stops at memrchr (5G2ONUzRgjY) on main, per @babyhuey's startup check on #1772: #1772 (comment). That's the condition Dean set for reopening this.

I had #1906 for the same import; yours came first, so I'll leave it to you. If you'd rather not pick it up, let me know and I'll reopen mine.