runtime: add nowritebarrierrec to funcs called at signal time

Also nosplit where needed.

Change-Id: I1e3f6f8f76df9ee7e87ed1b8560cef145928314c
Reviewed-on: https://go-review.googlesource.com/18395
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/sigqueue.go

@@ -180,6 +180,7 @@ func signal_ignored(s uint32) bool {
 // This runs on a foreign stack, without an m or a g.  No stack split.
 //go:nosplit
 //go:norace
+//go:nowritebarrierrec
 func badsignal(sig uintptr) {
 	cgocallback(unsafe.Pointer(funcPC(badsignalgo)), noescape(unsafe.Pointer(&sig)), unsafe.Sizeof(sig))
 }