[clang/mac] Accept -why_load and make -whyload an alias for it

From `man ld`:

     -why_load   Log why each object file in a static library is loaded.
                 That is, what symbol was needed.
                 Also called -whyload for compatibility.

`-why_load` is the spelling preferred by the linker and `-whyload` an old
compatibility setting. clang should accept the preferred form, and map both
forms to the preferred form.

Differential Revision: https://reviews.llvm.org/D98156