tests: Use clang-11 for the Kokoro compilation test
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 <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Maintainer: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5