selftests/powerpc: Add a signal fuzzer selftest

selftests/powerpc: Add a signal fuzzer selftest

This is a new selftest that raises SIGUSR1 signals and handles it in a
set of different ways, trying to create different scenario for testing
purpose.

This test works raising a signal and calling sigreturn interleaved
with TM operations, as starting, suspending and terminating a
transaction. The test depends on random numbers, and, based on them,
it sets different TM states.

Other than that, the test fills out the user context struct that is
passed to the sigreturn system call with random data, in order to make
sure that the signal handler syscall can handle different and invalid
states properly.

This selftest has command line parameters to control what kind of
tests the user wants to run, as for example, if a transaction should
be started prior to signal being raised, or, after the signal being
raised and before the sigreturn. If no parameter is given, the default
is enabling all options.

This test does not check if the user context is being read and set
properly by the kernel. Its purpose, at this time, is basically
guaranteeing that the kernel does not crash on invalid scenarios.

Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>