s390: Add DT_JMPREL pointing to .rela.[i]plt with static-pie

In static-pie case, there are IRELATIVE-relocs in
.rela.iplt (htab->irelplt), which will later be grouped
to .rela.plt.  On s390, the IRELATIVE relocations are
always located in .rela.iplt - even for non-static case.
Ensure that DT_JMPREL, DT_PLTRELA, DT_PLTRELASZ is added
to the dynamic section even if htab->srelplt->size == 0.
See _bfd_elf_add_dynamic_tags in bfd/elflink.c.

bfd/
	elf64-s390.c (elf_s390_size_dynamic_sections):
	Enforce DT_JMPREL via htab->elf.dt_jmprel_required.