Adds 64-bit POSIX support for AVX

- Adds unique enums for ymm registers to the ABI and the POSIX register context.
- Reworks the register context data structures to support a union of FXSAVE and XSAVE
--- Allows the same code base to deal with the FPU independent of the availability of AVX.
- Determine if AVX is supported by attempting to read XSAVE using ptrace.
--- Support an extended register set for avx registers if available.
- Provide a mechanism to assemble/parse register halves into a single ymm buffer for use with RegisterValue.
--- Reworked Read/WriteRegister routines to read/write/parse ymm registers.

Adds tests for ymm register write with read-back, and expressions involving ymm registers.
- Tests vary depending on the availability of an avx register set.

Thanks to Daniel and Matt for their reviews.

llvm-svn: 180572