cmd/go: add a -debug-runtime-trace flag

cmd/go: add a -debug-runtime-trace flag

The runtime/trace package proved useful for investigating go command
performance, and it makes sense (to me) to make this available for
development behind an undocumented flag, at the cost of ~25KB of binary
size. We could of course futher hide this functionality behind an
experiment or build tag, if necessary.

Updates #59157

Change-Id: I612320920ca935f1ee10bb6a803b7952f36c939b
Reviewed-on: https://go-review.googlesource.com/c/go/+/477896
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>