[AArch64] Fix test failures on elf configuration

[AArch64] Fix test failures on elf configuration

This patch fixed those failures on elf configuration by:

   * Improve the ILP32 target selector "aarch64_choose_ilp32_emul",
     makes it more robust. Target triples copied from configure.tgt

   * Updated emit-relocs-86/-overflow.d to use aarch64_choose_ilp32_emul
     which is following what have done with emit-relocs-28.

   * Those instruction encoding mismatch is because those encoding
     contains pc-relative address. As for elf, we may have different
     start address. relaxed encodind check, especially for
     aarch64-farcall-b/bl-plt, as the main purpose of those check are
     ELF text/data layout, we just want to make sure veneer to plt stub
     is generated.

2015-08-12  Jiong Wang  <jiong.wang@arm.com>

ld/testsuite/
  * ld-aarch64/aarch64-elf.exp (aarch64_choose_ilp32_emul): Support all
  four triple shapes: aarch64-*-linux*, aarch64-*-elf,
  aarch64_be-*-linux*, aarch64_be-*-elf.
  * ld-aarch64/emit-relocs-86.d: Use aarch64_choose_ilp32_emul.
  * ld-aarch64/emit-relocs-86-overflow.d: Likewise.
  * ld-aarch64/ld-aarch64/farcall-b-plt.d: Relax instrucion encoding
  check when they reflect address.
  * ld-aarch64/ld-aarch64/farcall-bl-plt.d: Likewise.