MIPS64: Fix TruncatingDiv.

- Fix the Macroassembler::TruncatingDiv to compute
  the correct result even when the magic number is negative.
- Fix the MULT instruction in the simulator.

TEST=mjsunit/compiler/division-by-constant
BUG=
R=dusan.milosavljevic@imgtec.com

Review URL: https://codereview.chromium.org/637273002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24475 ce2b1a6d-e550-0410-aec6-3dcde31c8c00