[lldb] [Process/NetBSD] Add register info for missing register sets

Add info for all register sets supported in NetBSD, particularly for all
registers 'expected' by LLDB.  This is necessary in order to fix
python_api/lldbutil/iter/TestRegistersIterator.py test that currently
fails due to missing names of register sets (None).

This copies fpreg descriptions from Linux, and combines Linux' AVX
and MPX registers into a single XState group, to fit NetBSD register
group design.  Technically, we do not support MPX registers
at the moment but gdb-remote insists on passing their errors anyway,
and if we do not include it in any group, they end up in a separate
anonymous group that breaks the test.

While at it, swap the enums for XState and DBRegs to match register set
ordering.

This also adds a few consts to the lldb-x86-register-enums.h to provide
more consistency between user registers and debug registers.

Differential Revision: https://reviews.llvm.org/D69667