runtime: fix newextram PC passed to race detector

PC passed to racegostart is expected to be a return PC
of the go statement. Race runtime will subtract 1 from the PC
before symbolization. Passing start PC of a function is wrong.
Add sys.PCQuantum to the function start PC.

Update #17190

Change-Id: Ia504c49e79af84ed4ea360c2aea472b370ea8bf5
Reviewed-on: https://go-review.googlesource.com/29712
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/proc.go

@@ -1438,7 +1438,7 @@ func oneNewExtraM() {
 	gp.lockedm = mp
 	gp.goid = int64(atomic.Xadd64(&sched.goidgen, 1))
 	if raceenabled {
-		gp.racectx = racegostart(funcPC(newextram))
+		gp.racectx = racegostart(funcPC(newextram) + sys.PCQuantum)
 	}
 	// put on allg for garbage collector
 	allgadd(gp)