Patch for gold internal error while fixing erratum 843419.

Patch for gold internal error while fixing erratum 843419.

The crash reason is that the insn to be moved to stub may be a
relocation spot, so instead of placing the origin insn (that is insn
before-relocation) to the stub, I have to place the relocated one.
Note the relocation involved is non-pc-relative, so it is safe to move
the relocated insn.

gold/ChangeLog:
2015-06-29  Han Shen  <shenhan@google.com>

    * AArch64.cc (Erratum_stub::Insn_utilities): New typedef.
    (Erratum_stub::update_erratum_insn): New method.
    (Stub_table::relocate_stubs): Modified to place relocated insn.
    (AArch64_relobj::fix_errata): Modified gold_assert.