[gdb/testsuite] Add have_mpx in lib/gdb.exp

The sources for the test-cases gdb.arch/i386-mpx*.exp contain have_mpx
functions that test whether the processor supports mpx instructions.

OTOH, the test-cases are compiled using -mmpx -fcheck-pointer-bounds, which
instrument all functions with mpx instructions.

So, the function that is supposed to test whether mpx instruction are
supported contains mpx instructions, which is a bit odd.

We could fix this by:
- factoring out the have_mpx function into a single source file, and
- compiling it without "-mmpx -fcheck-pointer-bounds".

But having the mpx support test as part of the test-cases seems like an
unnecessary complication that makes the test-cases more difficult to analyze,
reason about and modify.

So we go one step further and factor out the mpx support test in into a
gdb_caching_proc.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-01-12  Tom de Vries  <tdevries@suse.de>

	* gdb.arch/i386-mpx-call.c (have_mpx): Remove.
	(main): Remove call to have_mpx.
	* gdb.arch/i386-mpx-call.exp: Use have_mpx.
	* gdb.arch/i386-mpx-map.c (have_mpx): Remove.
	(main): Remote call to have_mpx.
	* gdb.arch/i386-mpx-map.exp: Use have_mpx.
	* gdb.arch/i386-mpx-sigsegv.c (have_mpx): Remove.
	(main): Remove call to have_mpx.
	* gdb.arch/i386-mpx-sigsegv.exp: Use have_mpx.
	* gdb.arch/i386-mpx-simple_segv.c (have_mpx): Remove.
	(main): Remove call to have_mpx.
	* gdb.arch/i386-mpx-simple_segv.exp: Use have_mpx.
	* gdb.arch/i386-mpx.c (have_mpx): Remove.
	(main): Remote call to have_mpx.
	* gdb.arch/i386-mpx.exp: Use have_mpx.
	* lib/gdb.exp (have_mpx): New proc.