[clang][Driver] Get darwin -Xarch_ working for subtypes, again.

35ca7d9d broke 471c4f82 for -arch flags that don't map 1:1
to the triple arch.  This has been broken for the many years since.
It hasn't mattered much since then, mostly because few people use it,
but also because it works for x86_64/i386, armv7/armv7s
don't differ much, arm64 is its own arch, and arm64/arm64_32 have
different arches (and it's a rare combination anyway).

But arm64/arm64e exposes this issue again.

Patch by: Justin Bogner <mail@justinbogner.com>
with some added tests.