The clang driver has a fairly fancy support for executing gcc instead of

clang itself. This dates back to clang's early days and while it looks like
some of it is still used (for kext for example), other parts are probably dead.

Remove the -ccc-clang-archs option and associated code. I don't think there
is any remaining setup where clang doesn't support an architecture but it can
expect an working gcc cross compiler to be available.

A nice side effect is that tests no longer need to differentiate architectures
that are included in production builds of clang and those that are not.

llvm-svn: 165545