Support external startup data in V8.

[Re-retry of r21696 and r21739]

If the embedder chooses, the 'natives' (library sources) and the
precompiled startup blob can be written to files during the build
process and handed over to V8 at startup. The main purpose would be
to reduce the size of the compiled binary for space constrained
platforms.

The build-time option is off by default. Nothing should change if
it's not enabled.

BUG=
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/334913004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21941 ce2b1a6d-e550-0410-aec6-3dcde31c8c00