Fix for failing mips stackwalker unittest.

Fix for failing mips stackwalker unittest.

This patch adds required change for stackwalker_mips_unittest to pass on 64bit
platforms.

One test from stackwalker_mips_unittest was failing as mentioned in:
https://breakpad.appspot.com/632002/
https://breakpad.appspot.com/633002/

BUG=none
TEST=stackwalker_mips_unittest (run on x86-64 and mips)

Commit URL: https://code.google.com/p/google-breakpad/source/detail?r=1218

[Ivan Penkov]

LGTM.