runtime: fix GODEBUG=schedtrace=X delay handling.

debug.schedtrace is an int32. Convert it to int64 before
multiplying with constant 1000000. Otherwise, schedtrace
values more than 2147 result in int32 overflow causing
incorrect delays between traces.

Change-Id: I064e8d7b432c1e892a705ee1f31a2e8cdd2c3ea3
Reviewed-on: https://go-review.googlesource.com/17712Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>

src/runtime/proc.go

@@ -3413,7 +3413,7 @@ func sysmon() {
 			lastscavenge = now
 			nscavenge++
 		}
-		if debug.schedtrace > 0 && lasttrace+int64(debug.schedtrace*1000000) <= now {
+		if debug.schedtrace > 0 && lasttrace+int64(debug.schedtrace)*1000000 <= now {
 			lasttrace = now
 			schedtrace(debug.scheddetail > 0)
 		}