S/390: Issue error for overflowing relocs.
    
    Building PIE executable from non-PIC code results in broken binaries.
    With this patch the problem is detected at link-time.
    
    bfd/ChangeLog:
    
    2017-01-07  Andreas Krebbel  <krebbel@linux.vnet.ibm.com>
    
    	* elf64-s390.c (elf_s390_relocate_section): Issue error for
    	non-PLT relocs of shared libary symbol in exectuable.