runtime: track the amount of scannable allocated stack for the GC pacer

runtime: track the amount of scannable allocated stack for the GC pacer

This change adds two fields to gcControllerState: stackScan, used for
pacing decisions, and scannableStackSize, which directly tracks the
amount of space allocated for inuse stacks that will be scanned.

scannableStackSize is not updated directly, but is instead flushed from
each P when at an least 8 KiB delta has accumulated. This helps reduce
issues with atomics contention for newly created goroutines. Stack
growth paths are largely unaffected.

StackGrowth-48			51.4ns ± 0%	51.4ns ± 0%	~	(p=0.927 n=10+10)
StackGrowthDeep-48		6.14µs ± 3%	6.25µs ± 4%	~	(p=0.090 n=10+9)
CreateGoroutines-48		273ns ± 1%	273ns ± 1%	~	(p=0.676 n=9+10)
CreateGoroutinesParallel-48	65.5ns ± 5%	66.6ns ± 7%	~	(p=0.340 n=9+9)
CreateGoroutinesCapture-48	2.06µs ± 1%	2.07µs ± 4%	~	(p=0.217 n=10+10)
CreateGoroutinesSingle-48	550ns ± 3%	563ns ± 4%	+2.41%	(p=0.034 n=8+10)

For #44167.

Change-Id: Id1800d41d3a6c211b43aeb5681c57c0dc8880daf
Reviewed-on: https://go-review.googlesource.com/c/go/+/309589
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>