Move C++ specific flags from C flags to C++ flags

This allows to compile non-C++ files in the v8 standalone build, e.g.
assembly files found in ICU.

R=jkummerow@chromium.org
BUG=v8:2745

Review URL: https://codereview.chromium.org/17638009

Patch from Jochen Eisinger <jochen@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15326 ce2b1a6d-e550-0410-aec6-3dcde31c8c00