Make it possible to compile d8 for the host toolset as well

BUG=v8:1775
R=jkummerow@chromium.org
LOG=y

Review URL: https://codereview.chromium.org/136763010

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18618 ce2b1a6d-e550-0410-aec6-3dcde31c8c00