[lldb] [Process/NetBSD] Use XStateRegSet for all FPU registers

Unify the x86 regset API to use XStateRegSet for all FPU registers,
therefore eliminating the legacy API based on FPRegSet.  This makes
the code a little bit simpler but most notably, it provides future
compatibility for register caching.

Since the NetBSD kernel takes care of providing compatibility with
pre-XSAVE processors, PT_{G,S}ETXSTATE can be used on systems supporting
only FXSAVE or even plain FSAVE (and unlike PT_{G,S}ETXMMREGS, it
clearly indicates that XMM registers are not supported).

Differential Revision: https://reviews.llvm.org/D90034