macOS: Do not pass Apple-specific flags to llvm-strip

Since !6164 (merged) we prefer llvm-strip over strip when using Clang. However, since !2892 (merged) on macOS we add flags -u -r, needed by Apple's strip for executables, but that llvm-strip does not need or support. Improve the condition to add Apple-specific flags only when the selected strip tool is Apple's.

Note that Apple dylibs must be stripped with -x with either Apple's strip or llvm-strip.

Fixes: #24601 (closed)