[S390] return address of compat signals

A 31-bit kernel always sets the high order bit in the return address
for a signal handler.
git commit d4e81b35b882d96f "[S390] allow all addressing modes" makes
sure that the high order bit is set in the signal return address for
standard signals of a 31-bit compat process but fails to do the same
for real-time signals. To make things consistent the bit needs to be
set by setup_rt_frame32 as well.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>