S390: Use DT_JUMPREL in prelink undo code.

On s390, the current prelink undo code in elf_machine_lazy_rel()
has the requirement, that the plt stubs use the first got slots
after the 3 reserved ones.

In case of undoing prelink, the plt got slots are reset to the correct
addresses whithin the corresponding plt-stub. Therefore the address
is calculated by the address of the first plt-stub-address which
was written by prelink (see l->l_mach.plt) to got[1] and index of
current relocation multiplied with 32 (=size of one plt slot).
The index was calculated with &current-got-slot - &got[3].

This patch removes the requirement, that the plt-got-slots are
starting at got[3]. The index is now calculated with
&current-reloc - &reloc[0]. The first struct Elf64_Rela is stored
at DT_JMPREL.

This patch is needed to prepare for partial relro support.

Ulrich Weigand suggested this approach to use DT_JMPREL - Thanks.

ChangeLog:

	* sysdeps/s390/linkmap.h (struct link_map_machine):
	Remove member gotplt and add member jmprel.
	* sysdeps/s390/s390-32/dl-machine.h
	(elf_machine_runtime_setup): Setup member jmprel with DT_JMPREL
	instead of gotplt with &got[3].
	(elf_machine_lazy_rel): Calculate address with reloc and jmprel.
	* sysdeps/s390/s390-64/dl-machine.h: Likewise.

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