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.

Change-Id: I97c1a73515d8d45988061c13a9488cea57ba7ecd
Reviewed-on: https://go-review.googlesource.com/45430
Reviewed-by: Than McIntosh <thanm@google.com>
diff --git a/libgo/go/runtime/proc.go b/libgo/go/runtime/proc.go
index 038f20e..32bc148 100644
--- a/libgo/go/runtime/proc.go
+++ b/libgo/go/runtime/proc.go
@@ -2251,6 +2251,7 @@
 	casgstatus(gp, _Grunning, _Gdead)
 	if isSystemGoroutine(gp) {
 		atomic.Xadd(&sched.ngsys, -1)
+		gp.isSystemGoroutine = false
 	}
 	gp.m = nil
 	gp.lockedm = nil