S/390: Fix claimfile failures

S/390: Fix claimfile failures

This fixes a segfault when trying to access the local_plt field in the
s390 specific elf data althoug the underlaying object is a generic elf
object.

This fixes the following testsuite failures:

< FAIL: plugin claimfile replace symbol with source
< FAIL: plugin claimfile resolve symbol with source
< FAIL: plugin claimfile replace file with source
< FAIL: plugin set symbol visibility with source
< FAIL: plugin ignore lib with source
< FAIL: plugin claimfile replace lib with source
< FAIL: plugin 2 with source lib
< FAIL: load plugin 2 with source
< FAIL: plugin 3 with source lib
< FAIL: load plugin 3 with source

bfd/ChangeLog:

2017-08-01  Andreas Krebbel  <krebbel@linux.vnet.ibm.com>

	* elf32-s390.c (elf_s390_finish_dynamic_sections): Skip if it
	isn't the S/390 specific elf data.
	* elf64-s390.c (elf_s390_finish_dynamic_sections): Likewise.