perf tests: Disable breakpoint accounting test for powerpc

We disable this test as instruction breakpoints (HW_BREAKPOINT_X) are
not available for powerpc.

Before applying patch:

21: Breakpoint accounting :
--- start ---
test child forked, pid 3635
failed opening event 0
failed opening event 0
watchpoints count 1, breakpoints count 0, has_ioctl 1, share 0
test child finished with -2
---- end ----
Breakpoint accounting: Skip

After applying patch:

21: Breakpoint accounting : Disabled

Signed-off-by: Sandipan Das <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Naveen N. Rao <[e-mail]>
Cc: Ravi Bangoria <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/tests/builtin-test.c

···
 	{
 		.desc = "Breakpoint accounting",
 		.func = test__bp_accounting,
+		.is_supported = test__bp_signal_is_supported,
 	},
 	{
 		.desc = "Number of exit events of a simple workload",