Introduces extended register sets whose availability can vary with the target processor.
- Includes a stub for AVX support in the x86-64 register context and a failing test for register sets that are unavailable.

Thanks to Greg Clayton for his review feedback.




git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@178252 91177308-0d34-0410-b5e6-96231b3b80d8