selftests/powerpc: Add alignment handler selftest

Add a selftest to exercise the powerpc alignment fault handler.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
[mpe: Add 32-bit support to the signal handler]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>