selftests/powerpc: Account for offline cpus in perf-hwbreak test

For systemwide tests, use online cpu mask to only open events on online
cpus. This enables this test to work on systems in lower SMT modes.

Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/15fd447dcefd19945a7d31f0a475349f548a3603.1669096083.git.naveen.n.rao@linux.vnet.ibm.com