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/cmd/compile/internal/gc/syntax.go

@@ -169,6 +169,7 @@ type Func struct {
 
 	Endlineno int32
 
+	Norace         bool // func must not have race detector annotations
 	Nosplit        bool // func should not execute on separate stack
 	Nowritebarrier bool // emit compiler error instead of write barrier
 	Dupok          bool // duplicate definitions ok