runtime: note custom GOMAXPROCS even if value doesn't change
When an application calls runtime.GOMAXPROCS(runtime.GOMAXPROCS(0)), the
runtime does not need to change the actual GOMAXPROCS value (via STW).
However, this call must still transition from "automatic" to "custom"
GOMAXPROCS state, thus disabling background updates.
Thus this case shouldn't return quite as early as it currently does.
Change-Id: I6a6a636c42f73996532bd9f7beb95e933256c9e7
Reviewed-on: https://go-review.googlesource.com/c/go/+/683815 Reviewed-by: Michael Knyszek <mknyszek@google.com> Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Pratt <mpratt@google.com>