Begin fixing Clang's predefined macros for various architectures. This

is *very* much a WIP that I'll be refining over the next several
commits, but I need to get this checkpoint in place for sanity.

This also adds a much more comprehensive test for architecture macros,
which is roughly generated by inspecting the behavior of a trunk build
of GCC. It still requires some massaging, but eventually I'll even check
in the script that generates these so that others can use it to append
more tests for more architectures, etc.

Next up is a bunch of simplification of the Targets.cpp code, followed
by a lot more test cases once we can reject invalid architectures.

llvm-svn: 140673