s390: Simplify test for absolute symbol

Simplify the test whether a symbol is absolute, by using the helper
bfd_is_abs_symbol.

bfd/
	* elf64-s390.c (elf_s390_relocate_section): Use
	bfd_is_abs_symbol to test whether symbol is absolute.

Signed-off-by: Jens Remus <jremus@linux.ibm.com>