gdb/arch: assert that X86_XSTATE_MPX is not set for x32

While trying to merge this commit:

  commit 4bb20a62
  Date:   Wed Mar 20 04:13:18 2024 -0700

      gdbserver: Clear X86_XSTATE_MPX bits in xcr0 on x32

With this patch series of mine:

  https://inbox.sourceware.org/gdb-patches/cover.1706801009.git.aburgess@redhat.com

I worried that there could be other paths that could result in an xcr0
value that has X86_XSTATE_MPX set in x32 mode.  As everyone eventually
calls amd64_create_target_description to build their target
description, I figured we could assert in here that if X86_XSTATE_MPX
is set then we should not be an x32 target, this should uncover any
other bugs in this area.

I'm not currently able to build/run any x32 binaries, so I have no way
to test this.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31511