runtime: eliminate write barriers from dropg

Currently this contains no write barriers because it's writing nil
pointers, but with the hybrid barrier, even these will produce write
barriers. However, since these are *gs and *ms, they don't need write
barriers, so we can simply eliminate them.

Updates #17503.

Change-Id: Ib188a60492c5cfb352814bf9b2bcb2941fb7d6c0
Reviewed-on: https://go-review.googlesource.com/31570Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/proc.go

@@ -2175,8 +2175,8 @@ top:
 func dropg() {
 	_g_ := getg()
 
-	_g_.m.curg.m = nil
-	_g_.m.curg = nil
+	setMNoWB(&_g_.m.curg.m, nil)
+	setGNoWB(&_g_.m.curg, nil)
 }
 
 func parkunlock_c(gp *g, lock unsafe.Pointer) bool {

src/runtime/runtime2.go

@@ -205,6 +205,14 @@ func (gp *guintptr) cas(old, new guintptr) bool {
 	return atomic.Casuintptr((*uintptr)(unsafe.Pointer(gp)), uintptr(old), uintptr(new))
 }
 
+// setGNoWB performs *gp = new without a write barrier.
+// For times when it's impractical to use a guintptr.
+//go:nosplit
+//go:nowritebarrier
+func setGNoWB(gp **g, new *g) {
+	(*guintptr)(unsafe.Pointer(gp)).set(new)
+}
+
 type puintptr uintptr
 
 //go:nosplit
@@ -221,6 +229,14 @@ func (mp muintptr) ptr() *m { return (*m)(unsafe.Pointer(mp)) }
 //go:nosplit
 func (mp *muintptr) set(m *m) { *mp = muintptr(unsafe.Pointer(m)) }
 
+// setMNoWB performs *mp = new without a write barrier.
+// For times when it's impractical to use an muintptr.
+//go:nosplit
+//go:nowritebarrier
+func setMNoWB(mp **m, new *m) {
+	(*muintptr)(unsafe.Pointer(mp)).set(new)
+}
+
 type gobuf struct {
 	// The offsets of sp, pc, and g are known to (hard-coded in) libmach.
 	sp   uintptr