Build "snapshot_blob.bin" only for the host when using separate toolsets.

BUG=389310
LOG=N
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/353143002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22068 ce2b1a6d-e550-0410-aec6-3dcde31c8c00