Global (FreeBSD): fixes build when libc defines memrchr as a macro

[pkubaj]

Summary

FreeBSD 16.0-CURRENT's <string.h> now provides C23-style
qualifier-preserving wrappers for the string search functions. Besides
the prototype, memrchr (like memchr, strchr, strstr, memmem,
...) is also defined as a function-like macro built on _Generic:

#define memrchr(b, c, n) __qualsel((b), \
    (const void *)(memrchr)((b), (c), (n)), (memrchr)((b), (c), (n)))

common/memrchr.h redeclares the function unconditionally. When
<string.h> has already been included, that declaration is expanded by
the macro and every translation unit fails with:

src/common/memrchr.h:11:7: error: expected identifier or '('
   11 | void* memrchr(const void* s, int c, size_t n);

A libc that defines memrchr as a macro has necessarily declared the
function it wraps, so skip our declaration in that case. Nothing
changes on platforms where memrchr is not a macro, and the fallback
implementation is still selected by check_function_exists.

Checklist

• I have tested my changes locally.