[AArch64] Improve PC-relative relocation check for shared library

[Nicholas Clifton]

Hi Jiong,

> 2015-04-16  Jiong. Wang  <jiong.wang@arm.com>
>
> bfd/
>    * elfnn-aarch64.c (elfNN_aarch64_final_link_relocate): Reject
>    PC-relative relocation for external symbol.
>
>
> ld/testsuite/
>    * ld-aarch64/pcrel.s: New testcase.
>    * ld-aarch64/pcrel_pic_defiend_local.d: New expect file.
>    * ld-aarch64/pcrel_pic_undefined.d: Ditto.
>    * ld-aarch64/aarch64-elf.exp: Run them.

Approved - please apply.

Cheers
   Nick