MIPS: Fix register usage in softfloat code path.

BUG=none
TEST=mjsunit/math-floor-part2.js

Review URL: https://codereview.chromium.org/12413032
Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14099 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
MIPS: Make the mips port build cleanly with clang.

Port r13583 (b39b116b) and also fix a couple mips-specific warnings.

BUG=

Review URL: https://codereview.chromium.org/13321002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14100 ce2b1a6d-e550-0410-aec6-3dcde31c8c00