Generate tests for all of the x86 SIMD instruction feature set

predefines based on the output of GCC as well as the CPU predefines.

Invert tests for __AVX__, Clang's AVX feature is hard coded off still.

Switch Atom from 'SSE3' to 'SSSE3'. This matches GCC's behavior, Intel's
documentation, and ICC's documentation (such as I could dig up).

Switch Athlon and Geode to enable 3dnowa rather than just 3dnow and
nothing (resp.).

llvm-svn: 140692