Improve handling of (X86) target features.

 - This is a WIP...

 - This adds -march= handling to the driver, and fixes the defaulting
   of -mcpu on Darwin (which was using the wrong test).

Instead of handling -m{sse, ...} in the driver, pass them to clang-cc as
 -target-feature [+-]name

In clang-cc, communicate with the (clang) target to discover the legal
features of a target, and the features which are enabled based on
-mcpu. This is currently hardcoded just enough to not be a feature
regression, we need to get this information from the backend's
TableGen information somehow.

This is used to construct the full list of features which are being
used, which is in turn used to initialize the predefines.

llvm-svn: 71061