runtime: push down systemstack requirement for tracer where possible
Currently lots of functions require systemstack because the trace buffer
might get flushed, but that will already switch to the systemstack for
the most critical bits (grabbing trace.lock). That means a lot of this
code is non-preemptible when it doesn't need to be. We've seen this
cause problems at scale, when dumping very large numbers of stacks at
once, for example.
This is a re-land of CL 572095 which was reverted in CL 577376. This
re-land includes a fix of the test that broke on the longtest builders.
Change-Id: Ia8d7cbe3aaa8398cf4a1818bac66c3415a399348
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/577377 Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Carlos Amedee <carlos@golang.org>