selftests/powerpc: Add a test of sigreturning to an unaligned address

Add a test of sigreturning to an unaligned address (low two bits set).
This should have no effect because the hardware will mask those bits.
However it previously falsely triggered a warning when
CONFIG_PPC_RFI_SRR_DEBUG=y.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211221135101.2085547-3-mpe@ellerman.id.au