Split out libplatform into a separate libary

Also remove the "use default platform" compile flag. Instead, the embedder
has to provide the platform.

Change all binaries to use the default platfrom from libplatform.

Unless --job-based-sweeping is passed, nothing uses the platform yet, so
nothing will break for embedders (yet).

BUG=none
R=jkummerow@chromium.org
LOG=y

Review URL: https://codereview.chromium.org/345903004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22180 ce2b1a6d-e550-0410-aec6-3dcde31c8c00