macos: workaround AppleClang compiler strrchr optimization bug

macos: workaround AppleClang compiler strrchr optimization bug

Apple shipped a clang built for macOS with an optimization enabled
that is not available on macOS. This compiler error may then be faced:

  ld: Undefined symbols:
	_memrchr, referenced from:
	    name_read(char const*) in map.cpp.o

This is a compiler error:

> On Mac OSX (macOS version 12.4, sdk version 12.1) llvm can replace call
> to strrchr() with call to memrchr() when string length is known at
> compile time. This results in link error, because memrchr is not present
> in libSystem.
> -- https://github.com/llvm/llvm-project/issues/62254

We workaround this by making the string length not known at build time
on macOS to avoid triggering the unavailable compiler optimization.