cmd/api: set compiler for all build contexts.

The generated syscall files for Windows are still breaking "go tool api"
(unknown function []byte); I'll look at fixing that separately.

Fixes #3285.

R=bradfitz
CC=golang-dev
https://golang.org/cl/5777062

src/cmd/api/goapi.go

@@ -52,6 +52,12 @@ var contexts = []*build.Context{
 	{GOOS: "windows", GOARCH: "386"},
 }
 
+func init() {
+	for _, c := range contexts {
+		c.Compiler = build.Default.Compiler
+	}
+}
+
 func contextName(c *build.Context) string {
 	s := c.GOOS + "-" + c.GOARCH
 	if c.CgoEnabled {