Non-CrOS builds should use clang for snapshot_toolchain, even on builds which use gcc for the target device. Change the default value for snapshot_toolchain to always use clang, except on ChromeOS gcc builds. In practice, every platform except ChromeOS always uses clang, so this should not affect any platform except non-CrOS gcc builds (like Cast)

BUG= internal b/30873074

Review-Url: https://codereview.chromium.org/2265983002
Cr-Commit-Position: refs/heads/master@{#38825}