Driver: Make macOS the default target OS for -arch arm64

This is a follow up to 565603cc,
which made macOS the default target OS for `-arch arm64` when
running on an Apple Silicon Mac. Now it'll be the default when
running on an Intel Mac too.

clang/test/Driver/apple-arm64-arch.c was a bit odd before: it was added
for the above commit, but tested the inverse behaviour and XFAIL'ed on
Apple Silicon. This inverts it to the (new) behaviour (that's now
correct regardless) and removes the XFAIL.

Radar-Id: rdar://90500294