s390/amode31: change type of __samode31, __eamode31, etc

For consistencs reasons change the type of __samode31, __eamode31,
__stext_amode31, and __etext_amode31 to a char pointer so they
(nearly) match the type of all other sections.

This allows for code simplifications with follow-on patches.

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>