gdb-15-branch: Clear the X86_XSTATE_MPX bit in XCRO for x32

commit 86888358
Author: Andrew Burgess <aburgess@redhat.com>
Date:   Sat Mar 23 16:17:36 2024 +0000

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

added

  if (xcr0 & X86_XSTATE_MPX)
    {
      /* MPX is not available on x32.  */
      gdb_assert (!is_x32);
      regnum = create_feature_i386_64bit_mpx (tdesc.get (), regnum);
    }

But x32 is a software convention.  There is no x32 mode in hardware and
CPU always returns the 64-bit mode XCR0 value for x32 processes.  This
regression was fixed on master branch by

commit bf616be9 (HEAD)
Author: Andrew Burgess <aburgess@redhat.com>
Date:   Thu Jan 25 14:25:57 2024 +0000

    gdb/gdbserver: share some code relating to target description creation

which used the gdbserver code to clear the X86_XSTATE_MPX bit in XCR0 for
x32.  Fix this regression on gdb-15-branch by clearing the X86_XSTATE_MPX
bit in XCR0 for x32 in gdb.

	PR gdb/32143
	* x86-linux-nat.c (x86_linux_nat_target::read_description): Clear
	the X86_XSTATE_MPX bit in XCR0 for x32.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>