[S390] 31 bit entry.S update.

Make the code in the 31 bit entry.S code as similar as possible to the
64 bit version in entry64.S. That makes it easier to add new code to
the first level interrupt handler that affects both 31 and 64 bit kernels.

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