s390: Avoid reloc overflows on undefined weak symbols

s390: Avoid reloc overflows on undefined weak symbols

Replace relative long addressing instructions of weak symbols, which
will definitely resolve to zero, with either a load address of 0, a
NOP, or a trapping insn.

This prevents the PC32DBL relocation from overflowing in case the
binary will be loaded at 4GB or more.

bfd/ChangeLog:

	* bfd/elf64-s390.c (elf_s390_relocate_section): Replace
	instructions using undefined weak symbols with relative addressing
	to avoid relocation overflows.

ld/ChangeLog:
	* ld/testsuite/ld-s390/s390.exp:
	* ld/testsuite/ld-s390/8GB.ld: New test.
	* ld/testsuite/ld-s390/weakundef-1.dd: New test.
	* ld/testsuite/ld-s390/weakundef-1.s: New test.