S/390: ifunc: Handle GOTOFF relocs on ifunc symbols.

S/390: ifunc: Handle GOTOFF relocs on ifunc symbols.

Normally a GOTOFF reloc only uses the GOT pointer to address something
relativ to it without actually requiring a GOT or PLT slot.  Things
change if the target is an ifunc symbol though.

bfd/ChangeLog:

	* elf32-s390.c (elf_s390_check_relocs): Fallthrough to the PLT
	slot allocating code for GOTOFF relocs on ifunc symbols.
	(elf_s390_gc_sweep_hook): Decrement plt refcount for GOTOFF relocs
	on ifunc symbols.
	(elf_s390_relocate_section): Redirect a GOTOFF reloc to an iplt
	slot.