race: cmd/cgo changes

This is a part of a bigger change that adds data race detection feature:
https://golang.org/cl/6456044
This change breaks circular dependency between runtime/race and syscall packages.

R=rsc
CC=golang-dev
https://golang.org/cl/6498079

src/cmd/cgo/main.go

@@ -147,6 +147,7 @@ var gccgo = flag.Bool("gccgo", false, "generate files for use with gccgo")
 var gccgoprefix = flag.String("gccgoprefix", "", "-fgo-prefix option used with gccgo")
 var gccgopkgpath = flag.String("gccgopkgpath", "", "-fgo-pkgpath option used with gccgo")
 var importRuntimeCgo = flag.Bool("import_runtime_cgo", true, "import runtime/cgo in generated code")
+var importSyscall = flag.Bool("import_syscall", true, "import syscall in generated code")
 var goarch, goos string
 
 func main() {

src/cmd/cgo/out.go

@@ -51,12 +51,16 @@ func (p *Package) writeDefs() {
 	fmt.Fprintf(fgo2, "// Created by cgo - DO NOT EDIT\n\n")
 	fmt.Fprintf(fgo2, "package %s\n\n", p.PackageName)
 	fmt.Fprintf(fgo2, "import \"unsafe\"\n\n")
+	if *importSyscall {
 		fmt.Fprintf(fgo2, "import \"syscall\"\n\n")
+	}
 	if !*gccgo && *importRuntimeCgo {
 		fmt.Fprintf(fgo2, "import _ \"runtime/cgo\"\n\n")
 	}
 	fmt.Fprintf(fgo2, "type _ unsafe.Pointer\n\n")
+	if *importSyscall {
 		fmt.Fprintf(fgo2, "func _Cerrno(dst *error, x int) { *dst = syscall.Errno(x) }\n")
+	}
 
 	typedefNames := make([]string, 0, len(typedef))
 	for name := range typedef {