tests: Use clang-11 for the Kokoro compilation test

We support up to clang-11 which includes some additional checks. It's
better to catch these during the Kokoro/quick tests than during the
nightly compilation tests.

Change-Id: I8076f296875448d39416fecc947f71230ce2ddf7
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/56003
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
1 file changed