[lldb] [Process/FreeBSDRemote] Support YMM reg via PT_*XSTATE

Add a framework for reading/writing extended register sets via
PT_GETXSTATE/PT_GETXSTATE_INFO/PT_SETXSTATE, and use it to support
YMM0..YMM15.  The code is prepared to handle arbitrary XSAVE extensions,
including correct offset handling.

This fixes Shell/Register/*ymm* tests.

Differential Revision: https://reviews.llvm.org/D89193