[lldb] [lit] Add feature flags for native CPU features

Add a new lit-cpuid tool that detects CPU features used by some of
the tests, and use it to populate available_features in lit.  For now,
this means that the test for MM/XMM register read will be run only
when the host CPU support SSE instruction set.  However, this is going
to make it possible to introduce additional tests relying on AVX.

Differential Revision: https://reviews.llvm.org/D61073

llvm-svn: 359303