cmd/cgo: print the builtin prolog after the per-file preamble

The preamble may want to #define some special symbols
and then #include <sys/types.h> itself. The builtin prolog
also #includes <sys/types.h>, which would break such a
preamble (because the second #include will be a no-op).

The use of sys/types.h in the builtin prolog is new since Go 1.1,
so this should preserve the semantics of more existing cgo
code than we would otherwise.

It also fixes src/pkg/syscall/mkall.sh's use of go tool cgo -godefs
on some Linux systems.

Thanks to fullung@ for identifying the problem.

Fixes #6558.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/14684044

src/cmd/cgo/gcc.go

@@ -188,8 +188,8 @@ func (p *Package) Translate(f *File) {
 // in the file f and saves relevant renamings in f.Name[name].Define.
 func (p *Package) loadDefines(f *File) {
 	var b bytes.Buffer
-	b.WriteString(builtinProlog)
 	b.WriteString(f.Preamble)
+	b.WriteString(builtinProlog)
 	stdout := p.gccDefines(b.Bytes())
 
 	for _, line := range strings.Split(stdout, "\n") {
@@ -301,8 +301,8 @@ func (p *Package) guessKinds(f *File) []*Name {
 	}
 
 	var b bytes.Buffer
-	b.WriteString(builtinProlog)
 	b.WriteString(f.Preamble)
+	b.WriteString(builtinProlog)
 	b.WriteString("void __cgo__f__(void) {\n")
 
 	// For a #defined expression, clang silences the warning about "unused expression".
@@ -417,8 +417,8 @@ func (p *Package) loadDWARF(f *File, names []*Name) {
 	// for each entry in names and then dereference the type we
 	// learn for __cgo__i.
 	var b bytes.Buffer
-	b.WriteString(builtinProlog)
 	b.WriteString(f.Preamble)
+	b.WriteString(builtinProlog)
 	for i, n := range names {
 		fmt.Fprintf(&b, "typeof(%s) *__cgo__%d;\n", n.C, i)
 		if n.Kind == "const" {