Cherry picked debug info fix from upstream MCLinker.

For the issue fixed by this patch see the discussion here (https://groups.google.com/forum/#!topic/mclinker/WiVvcnX7QdA).
Picked from commit (https://github.com/mclinker/mclinker/commit/9da046f3cd0702704713d7935814b3eb26072268).

Original message:
debug info: Fix debug string offset

Also update Mips64 regression test

Conflicts:
    test/Mips/dso/libfoo64.golden.so

Change-Id: I9e6ca2691ebfa47db5b0c71689126f4e69e940b8