s390/boot: make type of __vmlinux_relocs_64_start|end consistent

Make the type of __vmlinux_relocs_64_start|end symbols as
char array, just like it is done for all other sections.
Function rescue_relocs() is simplified as result.

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