[gdb/testsuite] Fix i386-mpx.exp compilation warnings

[gdb/testsuite] Fix i386-mpx.exp compilation warnings

When running test-case gdb.arch/i386-mpx.exp with gcc-10, we get:
...
Running src/gdb/testsuite/gdb.arch/i386-mpx.exp ...
gdb compile failed, xgcc: warning: switch '-mmpx' is no longer supported
xgcc: warning: switch '-fcheck-pointer-bounds' is no longer supported
...

The test-case uses a combination of options, -mmpx and -fcheck-pointer-bounds.

The -fcheck-pointer-bounds option requires the -mmpx option:
...
$ gcc -fcheck-pointer-bounds ~/hello.c
hello.c:1:0: warning: Pointer Checker requires MPX support on this target. \
  Use -mmpx options to enable MPX.
 #include <stdio.h>

cc1: error: ‘-fcheck-pointer-bounds’ is not supported for this target
...

Both options is no longer supported in gcc-9.

Fix the warnings by testing if the option combination is supported.

Tested on x86_64-linux, with gcc-7.5.0 and gcc-10.0.1.

gdb/testsuite/ChangeLog:

2020-05-02  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (supports_mpx_check_pointer_bounds): New proc.
	* gdb.arch/i386-mpx-call.exp: Use supports_mpx_check_pointer_bounds.
	* gdb.arch/i386-mpx-map.exp: Same.
	* gdb.arch/i386-mpx-sigsegv.exp: Same.
	* gdb.arch/i386-mpx-simple_segv.exp: Same.
	* gdb.arch/i386-mpx.exp: Same.