tests,misc: Added gem5.fast clang compilation to Kokoro

tests,misc: Added gem5.fast clang compilation to Kokoro

Compilation issues in Clang and in compiling gem5.fast are normally
only caught during gem5's weekly, intensive, compilation checks:
http://jenkins.gem5.org/job/Compiler-Checks. The purpose of this change
is to have smaller checks on every commit, reducing the chance of
uncompilable code being submitted.

Change-Id: Idd8c6795ff73e21b1814281c31fc7ae39f09dcc5
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/37478
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>