Don't use AVX/XSTATE API on Windows.

CopyContext is necessary to safely get the XState, but LLDB doesn't currently
use the XState. CopyContext is available as of Windows 7 SP1, so it can't be
used on Vista.  Furthermore, it requires the Windows 8 SDK it compile,
making the baseline for compiling and running LLDB higher than necessary.

Patch by: Adrian McCarthy
Reviewed by: Zachary Turner
Differential Revision: http://reviews.llvm.org/D7572

llvm-svn: 229710