runtime: mark dropm and callees nowritebarrierrec

The CgocallbackDone function calls dropm after it calls entersyscall,
    which means that dropm must not have any write barriers. Mark it
    accordingly.
    
    Reviewed-on: https://go-review.googlesource.com/46464

From-SVN: r249577

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-55b2b2dee55bd495a770347daaf858634057ecb0
+c5aa42107d116c9121de447d214297c88dd9f5fa
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/os_gccgo.go

@@ -32,6 +32,7 @@ func minit() {
 
 // Called from dropm to undo the effect of an minit.
 //go:nosplit
+//go:nowritebarrierrec
 func unminit() {
 	unminitSignals()
 }

libgo/go/runtime/proc.go

@@ -1447,6 +1447,9 @@ func oneNewExtraM() {
 // in which dropm happens on each cgo call, is still correct too.
 // We may have to keep the current version on systems with cgo
 // but without pthreads, like Windows.
+//
+// CgocallBackDone calls this after releasing p, so no write barriers.
+//go:nowritebarrierrec
 func dropm() {
 	// Clear m and g, and return m to the extra list.
 	// After the call to setg we can only call nosplit functions
@@ -1492,6 +1495,7 @@ var extraMWaiters uint32
 // return a nil list head if that's what it finds. If nilokay is false,
 // lockextra will keep waiting until the list head is no longer nil.
 //go:nosplit
+//go:nowritebarrierrec
 func lockextra(nilokay bool) *m {
 	const locked = 1
 
@@ -1524,6 +1528,7 @@ func lockextra(nilokay bool) *m {
 }
 
 //go:nosplit
+//go:nowritebarrierrec
 func unlockextra(mp *m) {
 	atomic.Storeuintptr(&extram, uintptr(unsafe.Pointer(mp)))
 }

libgo/go/runtime/signal_unix.go

@@ -599,6 +599,7 @@ func minitSignalMask() {
 // unminitSignals is called from dropm, via unminit, to undo the
 // effect of calling minit on a non-Go thread.
 //go:nosplit
+//go:nowritebarrierrec
 func unminitSignals() {
 	if getg().m.newSigstack {
 		signalstack(nil, 0)