[S390] Handling of 4096 bit RSA keys in CRT format.

Also process 4096 bit RSA keys in CRT format. Handle them like the
smaller keys and take care of the zero padding.

Signed-off-by: Felix Beck <felix.beck@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>