runtime/cgo: make compatible with race detector

Some routines run without and m or g and cannot invoke the
race detector runtime. They must be opaque to the runtime.
That used to be true because they were written in C.
Now that they are written in Go, disable the race detector
annotations for those functions explicitly.

Add test.

Fixes #10874.

Change-Id: Ia8cc28d51e7051528f9f9594b75634e6bb66a785
Reviewed-on: https://go-review.googlesource.com/12534
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/sigqueue.go

@@ -164,6 +164,7 @@ func signal_ignore(s uint32) {
 
 // This runs on a foreign stack, without an m or a g.  No stack split.
 //go:nosplit
+//go:norace
 func badsignal(sig uintptr) {
 	cgocallback(unsafe.Pointer(funcPC(badsignalgo)), noescape(unsafe.Pointer(&sig)), unsafe.Sizeof(sig))
 }