[AArch64] Generate DT_TEXTREL for relocation against read-only section

[AArch64] Generate DT_TEXTREL for relocation against read-only section

2015-06-23  Jiong Wang  <jiong.wang@arm.com>

  bfd/
    * elfnn-aarch64.c (aarch64_readonly_dynrelocs): New function.
    (elfNN_aarch64_size_dynamic_sections): Traverse hash table to check
    relocations against read-only sections.

  ld/testsuite/
    * ld-aarch64/dt_textrel.s: New testcase.
    * ld-aarch64/dt_textrel.d: New expectation file.
    * ld-aarch64/aarch64-elf.exp: Run new testcase.