runtime: create wrappers for gcDrain to identify time in profiles

runtime: create wrappers for gcDrain to identify time in profiles

Currently it's impossible to identify in profiles where gcDrain-related
time is coming from. More specifically, what kind of worker. Create
trivial wrappers for each worker so that the difference shows up in
stack traces.

Also, clarify why gcDrain disables write barriers.

Change-Id: I966e3c0b1c583994e691f486bf0ed8cabb91dbbb
Reviewed-on: https://go-review.googlesource.com/c/go/+/521815
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>