Currently, snapshot_toolchain is hardcoded to use a clang host
toolchain. Use a GCC toolchain if is_clang is false.

Revert this when this is root-caused (see crbug.com/601486)

LOG=Y
BUG=601486

Review URL: https://codereview.chromium.org/1809643003

Cr-Commit-Position: refs/heads/master@{#35341}