MIPS: Fix mips gyp build for chromium.

Our planned CL for the Chromium build system passes the toolchain
vars though the Makefiles, so that we do not depend on $CXX, etc.,
in the environment.

That causes a problem with deriving the 'm32flag' in v8 gyp.

This patch lets us build with or without the toolchain vars in
the environment.

BUG=

Review URL: https://codereview.chromium.org/12218113
Patch from Paul Lind <plind44@gmail.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13695 ce2b1a6d-e550-0410-aec6-3dcde31c8c00