Refactor NativeRegisterContextLinux_x86_64 code.
This patch refactors the way the XState type is checked and, in order to
simplify the code, it removes the usage of the 'cpuid' instruction: just checking
if the ptrace calls done throuhg ReadFPR is enough to verify both if there is
HW support and if there is kernel support. Also the XCR0 bits are enough to check if
there is both HW and kernel support for AVX and MPX.
Differential Revision: https://reviews.llvm.org/D24764