Turn ASSERTs in platform setup releated code into CHECKs

This way, we will already catch misconfigurations in release builds,
instead of getting random bug reports way later during runtime.

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

Review URL: https://codereview.chromium.org/429203002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22734 ce2b1a6d-e550-0410-aec6-3dcde31c8c00