x86 synthetic plt symbols

x86 synthetic plt symbols

Changing "pushq $1" in the following to "pushq $too_big" results in an
abort.  BFD shouldn't abort on (deliberately) bad user input.

 400480:       ff 25 9a 0b 20 00       jmpq   *0x200b9a(%rip)
 400486:       68 01 00 00 00          pushq  $0x1
 40048b:       e9 d0 ff ff ff          jmpq   400460 <_init+0x20>

	* elf64-x86-64.c (elf_x86_64_get_plt_sym_val): Don't abort on
	an out of range reloc_index.
	* elf32-i386.c (elf_i386_get_plt_sym_val): Likewise.