cmd/go: add a debug-trace flag to generate traces

If cmd/go is provided with a -debug-trace=<file> option, cmd/go will write an
execution trace to that file.

Updates #38714

Change-Id: I3e6521343902c08266a0292f4280298a3bf8b725
Reviewed-on: https://go-review.googlesource.com/c/go/+/237683


Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>