runtime: use compileCallback for ctrlhandler

This replaces the externalthreadhandler-based implementation of
ctrlhandler with one based on compileCallback. This is a step toward
eliminating externalthreadhandler.

For #45530.

Change-Id: I2de2f2f37777af292db67ccf8057b7566aab81f8
Reviewed-on: https://go-review.googlesource.com/c/go/+/309632


Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>