Fix false FAIL on gdb.base/stap-probe.exp, due to ICF optimization

GCC 6's ICF optimization pass is making the declaration of 'm1' and
'm2', on gdb.base/stap-probe.c, to be unified.  However, this leads to
only one instance of the probe 'two' being created, which causes a
failure on the testsuite (which expects a multi-location breakpoint to
be inserted on the probe).

This patch fixes this failure by declaring a dummy variable on 'm1',
and using it as an argument to m1's version of probe 'two'.  Since we
do not care about the contents of the functions nor about the
arguments of each probe 'two', this is OK.

gdb/testsuite/ChangeLog:
2016-09-11  Sergio Durigan Junior  <sergiodj@redhat.com>
	    Jan Kratochvil  <jan.kratochvil@redhat.com>

	* gdb.base/stap-probe.c (m1): New variable 'dummy', necessary to
	make m1's definition to be different from m2's.  Use 'dummy' as an
	argument for probe 'two'.