s390/kexec: Move early_pgm_check_handler() to text section

The early_pgm_check_handler() function is also used after the
init phase in s390_reset_system(). Therefore it must not be in
the init section.

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>