selftests/powerpc: Skip tm-trap if transactional memory is not enabled

Some processor revisions do not support transactional memory, and
additionally kernel support can be disabled. In either case the
tm-trap test should be skipped, otherwise it will fail with a SIGILL.

Fixes: a08082f8 ("powerpc/selftests: Check endianness on trap in TM")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>