[lldb] [test] Make AVX/MPX register tests more robust and fix on BSD

Make the AVX/MPX register tests more robust by checking for the presence
of actual registers rather than register sets.  Account for the option
that the respective registers are defined but not available, as is
the case on FreeBSD and NetBSD.  This fixes test regression on these
platforms.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.llvm.org/D128041