runtime: don't use trace.lock for trace reader parking
We're about to require that all uses of trace.lock be on the system
stack. That's mostly easy, except that it's involving parking the
trace reader. Fix this by changing that parking protocol so it instead
synchronizes through an atomic.
For #53979.
Change-Id: Icd6db8678dd01094029d7ad1c612029f571b4cbb
Reviewed-on: https://go-review.googlesource.com/c/go/+/418955
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>