s390/traps,mm: add conditional trap handlers

Add conditional trap handlers similar to conditional system calls
(COND_SYSCALL), to reduce the number of ifdefs.

Trap handlers which may or may not exist depending on config options
are supposed to have a COND_TRAP entry, which redirects to
default_trap_handler() for non-existent trap handlers during link
time.

This allows to get rid of the secure execution trap handlers for the
!PGSTE case.

Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>