iai-callgrind now correctly exits with error if regressions were found
[1], so we no longer need to check for regressions manually. Remove this
check and instead exit based on the exit status of the benchmark run.
[1] https://github.com/iai-callgrind/iai-callgrind/issues/337
We don't actually need this for now, but eventually it would be nice to
run icount benchmarks on multiple targets. Start tagging artifact names
with the architecture, and allow passing `--tag` to `ci-util.py` in
order to retrieve the correct one.
Many contributions to compiler-builtins don't have any need to touch
libm, and could get by with the few minutes of CI for compiler-builtins
rather than the ~30 minutes for libm. We already have some scripts that
handle changed file detection, so expand its use to skip libm CI if it
doesn't need to run.
