gdbserver: Clear X86_XSTATE_MPX bits in xcr0 on x32

Since MPX isn't available for x32, we should clear X86_XSTATE_MPX bits
on x32.

	PR server/31511
	* linux-x86-low.cc (x86_linux_read_description): Clear
	X86_XSTATE_MPX bits in xcr0 on x32.
Reviewed-by: Felix Willgerodt <felix.willgerodt@intel.com>