Fix floating point register reads x86_64 linux on targets with no AVX support

Summary:
On for 64-bit targets, the correct register set to read the fxsave are is
NT_PRFPREG (only 32-bit targets need NT_PRXFPREG, presumably for historic
reasons). Reference:
<https://github.com/torvalds/linux/blob/v4.8/arch/x86/kernel/ptrace.c#L1261>.

Reviewers: tberghammer, valentinagiusti

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D27161

llvm-svn: 288038