Revert of Use GCC for snapshot_toolchain when is_clang=false. (patchset #1 id:1 of https://codereview.chromium.org/1809643003/ )

Reason for revert:
Blocks roll: https://codereview.chromium.org/1874173002/

Original issue's description:
> Use GCC for snapshot_toolchain when is_clang=false.
>
> 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
>
> Committed: https://crrev.com/920370d1a910681bf464bd91aced316058ea2f6c
> Cr-Commit-Position: refs/heads/master@{#35341}

TBR=jochen@chromium.org,alokp@chromium.org,dpranke@chromium.org,slan@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=601486
LOG=Y

Review URL: https://codereview.chromium.org/1873113002

Cr-Commit-Position: refs/heads/master@{#35372}