x86/mpx: Remove selftests Makefile entry

MPX is being removed from the kernel due to a lack of support in the
toolchain going forward (gcc).

This is the smallest possible patch to fix some issues that have been
reported around running the MPX selftests.  It it would also have been part
of any removal series, it is offered first.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190705175318.784C233E@viggo.jf.intel.com