cgo: write _cgo_export.h to object directory, not source dir

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5540048

src/cmd/cgo/out.go

@@ -396,7 +396,7 @@ func (p *Package) writeOutputFunc(fgcc *os.File, n *Name) {
 // from Go so that they are callable from C.
 func (p *Package) writeExports(fgo2, fc, fm *os.File) {
 	fgcc := creat(*objDir + "_cgo_export.c")
-	fgcch := creat("_cgo_export.h")
+	fgcch := creat(*objDir + "_cgo_export.h")
 
 	fmt.Fprintf(fgcch, "/* Created by cgo - DO NOT EDIT. */\n")
 	fmt.Fprintf(fgcch, "%s\n", gccExportHeaderProlog)

src/pkg/go/build/build.go

@@ -396,8 +396,7 @@ func (b *build) cgo(cgofiles, cgocfiles []string) (outGo, outObj []string) {
 		Output: output,
 	})
 	outGo = append(outGo, gofiles...)
-	exportH := filepath.Join(b.path, "_cgo_export.h")
-	b.script.addIntermediate(defunC, exportH, b.obj+"_cgo_flags")
+	b.script.addIntermediate(defunC, b.obj+"_cgo_export.h", b.obj+"_cgo_flags")
 	b.script.addIntermediate(cfiles...)
 
 	// cc _cgo_defun.c