selftest/powerpc: Add test for sigreturn in transaction

Ensure that kernel is throwing away the suspended transaction when
sigreturn() is called otherwise it if fails to restore the signal
frame's TM SPRS.

Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Reviewed-by: Cyril Bur <cyrilbur@gmail.com>
[mpe: Add have_htm() check, minor formatting, add SPDX tag]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>