swift234

When targeting arm64-apple-ios, the target CPU is cyclone,
which provides the Neon feature.  Do all the necessary
plumbing to get this from the driver to the backend.

Also, support -arch arm64, and diagnose bad -arch values
instead of silently ignoring them.  It's not clear to me
that we really want to support -arch as an alternative
to -target, but unless we rip it out or establish some
sort of real policy about it, it really ought to do
something approximating the right thing.

It would be nice if we could abstract enough of clang's
driver that we could re-use some of its basic logic about
tool chains and targets instaed of iteratively
rediscovering everything it does that's actually
critically important.

Swift SVN r16447