runtime: move tSweepTerm capture closer to STW

tSweepTerm and pauseStart are supposed to be when STW was triggered,
but right now they're captured a bit before STW. Move these down to
immediately before we trigger STW.

Fixes #19590.

Change-Id: Icd48a5c4d45c9b36187ff986e4f178b5064556c1
Reviewed-on: https://go-review.googlesource.com/49612
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/mgc.go

@@ -1248,13 +1248,13 @@ func gcStart(mode gcMode, trigger gcTrigger) {
 
 	gcResetMarkState()
 
-	now := nanotime()
 	work.stwprocs, work.maxprocs = gcprocs(), gomaxprocs
-	work.tSweepTerm = now
 	work.heap0 = atomic.Load64(&memstats.heap_live)
 	work.pauseNS = 0
 	work.mode = mode
 
+	now := nanotime()
+	work.tSweepTerm = now
 	work.pauseStart = now
 	systemstack(stopTheWorldWithSema)
 	// Finish sweep before we start concurrent scan.