cmd/bench: benchmark baseline toolchain
cmd/bench: benchmark baseline toolchain

If BENCH_BASELINE_GOROOT is set, additionally benchmark that toolchain.
The benchfmt label 'toolchain' differentiates the 'experiment' and
'baseline' toolchains.

For golang/go#49207.

Change-Id: I737fa56786dc482172942462c5776c4c2773c0c5
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/376096
Trust: Michael Pratt <e7f1edc6d5aa9f6c8c377425db8f1a5816cbbb88@google.com>
Run-TryBot: Michael Pratt <e7f1edc6d5aa9f6c8c377425db8f1a5816cbbb88@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>

golang/benchmarks,golang/benchmarks