* Make C++ toolchain optional

Currently, if you transition a go_binary, you must have a C++ toolchain
for it even if it's building in pure mode without cgo.

By making this toolchain optional, we push the error from a
toolchain-resolution-time error to an analysis-time error only if a
toolchain is both _needed_ and not found.

* Add tests

* Add target that actually uses cgo

* Switch to use bazel_features

Also switch from @bazel_tools//tools/cpp:current_cc_toolchain to
@bazel_tools//tools/cpp:optional_current_cc_toolchain because otherwise
we still get a hard error if the toolchain can't be found.

* Review comments

* Fix a race detected only if a test times out

I believe this can happen because the goroutine is started before the
command is started. Start the process first, then on success, start the
signal handler. Add a test for regression.

I verified the test fails without my change to wrap.go and the race is
from the signal handling goroutine.

* Register signal handler before cmd.Start()