Improve test for processor feature om MPX registers test.

Improve test for processor feature om MPX registers test.

Skips the MPX register test in case target is not Intel.
Improves the test for MPX feature making MPX and AVX512
tests more similar in terms of initialization.
Indentation was improved on sample file and final return added
to have_mpx.  On test file identation was improved and
gdb_send was exchanged by gdb_test_multiple.

2015-06-08  Walfred Tedeschi  <walfred.tedeschi@intel.com>

gdb/testsuite
	* gdb.arch/i386-mpx.c: Added final return to the have_mpx
	function and improved indentation.
	* gdb.arch/i386-mpx.exp: Exchanging gdb_send and gdb_expect for
	gdb_test_multiple. Added additional tests to skip the test.