runtime: clear isSystemGoroutine in goexit

Otherwise it may be set when the g struct is reused via gfput/gfget.
    
    Test is golang.org/x/net/http2 with GOMAXPROCS=12.
    
    Reviewed-on: https://go-review.googlesource.com/45430

From-SVN: r249143

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-908fc7e46ebe36658ed86b65a3d165fccb2e8576
+3b44ad058abda0d1b0b6c928987270da50ab7431
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/proc.go

@@ -2251,6 +2251,7 @@ func goexit0(gp *g) {
 	casgstatus(gp, _Grunning, _Gdead)
 	if isSystemGoroutine(gp) {
 		atomic.Xadd(&sched.ngsys, -1)
+		gp.isSystemGoroutine = false
 	}
 	gp.m = nil
 	gp.lockedm = nil