perf test: Disable breakpoint signal tests for powerpc

The following tests are failing on powerpc:

  # perf test break
  18: Breakpoint overflow signal handler  : FAILED!
  19: Breakpoint overflow sampling        : FAILED!

The powerpc kenel so far does not have support to even create
instruction breakpoints using the perf event interface, so those tests
fail early in the config phase.

I added a '->is_supported()' callback to test struct to be able to
disable specific tests. It seems better than putting ifdefs directly to
the test array.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20170601205450.GA398@krava


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>