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.