[lldb] [Process/NetBSD] Support reading YMM registers via PT_*XSTATE

Provide a (conditional) support for the new PT_GETXSTATE
and PT_SETXSTATE ptrace() requests, and use them to implement getting
and setting YMM registers.  The functions used for splitting
and recombining YMM register data are based on matching functions
in FreeBSD plugin, with some simplification and updates to match NetBSD
structures.

Differential Revision: https://reviews.llvm.org/D63545

llvm-svn: 364779