runtime: don't do a direct G handoff in semrelease on systemstack
semrelease is safe to call on the system stack (since it just readies
goroutines) except for the fact that it might perform a direct G
handoff and call into the scheduler. If handoff is set to false this is
exceptionally rare, but could happen, and has happened for the trace
reader goroutine which releases a trace.doneSema.
Fixes #73469.
Change-Id: I37ece678bc4721bbb6e5879d74daac762b7d742a
Reviewed-on: https://go-review.googlesource.com/c/go/+/680315
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Carlos Amedee <carlos@golang.org>