selftests/powerpc: Add TM signal return test

Test the kernel's signal return code to ensure that it doesn't crash
when both the transactional and suspend MSR bits are set in the signal
context.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Tested-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
[mpe: Skip if we don't have TM]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>