This updates the existing bootstrap.sh script for gcmole to work against
LLVM and Clang version 8.0 releases. This is a follow-up to a previous
change which adapted the gcmole plugin to compile against those same
versions.

R=mslekova@chromium.org
BUG=v8:8813

Change-Id: Id6052fb9a7ec8a63d205eab2d4e233e2121c733d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1511275Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60116}