MIPS: Fix abs_d instruction in the simulator.

The abs_d instruction was implemented wrongly in the simulator,
it doesn't reverse the sign of the -0 number.

This commit fixes the abs_d instruction implementation.

TEST=msjunit/math-abs

BUG=

Review URL: https://codereview.chromium.org/15906014

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15053 ce2b1a6d-e550-0410-aec6-3dcde31c8c00