S/390: Fix (some) PIE+undef weak failures

S/390: Fix (some) PIE+undef weak failures

This fixes these failures on 64 bit which currently occur when running
the Binutils testsuite with a default PIE compiler.

< FAIL: Build rdynamic-1
< FAIL: Build dynamic-1
< FAIL: Build pr22269-1

bfd/ChangeLog:

2017-12-13  Andreas Krebbel  <krebbel@linux.vnet.ibm.com>

	* elf64-s390.c (elf_s390_adjust_dynamic_symbol): Use
	UNDEFWEAK_NO_DYNAMIC_RELOC.
	(allocate_dynrelocs): Likewise.
	(elf_s390_relocate_section): Check resolved_to_zero.
	(elf_s390_finish_dynamic_symbol): Don't generate runtime reloc if
	UNDEFWEAK_NO_DYNAMIC_RELOC.