[lldb] [Process/FreeBSDRemote] Fix regset names and related tests

Restore Linux-alike regset names for AVX/MPX registers
as TestLldbGdbServer seems to depend on them.  At the same time, fix
TestRegisters to be aware that they are not available on FreeBSD
and NetBSD, at least until we figure out a better way of reporting
unsupported register sets.

Differential Revision: https://reviews.llvm.org/D91923