cmd/cgo: stop expanding typedefs once we reach __builtin types

Expanding __builtin types (__builtin_va_list, particularly) leads
to problems because they are expanded by the compiler itself - the
expansions are not generated by anything in a .h file. The types
a __builtin type expand to are thus very confusing to cgo.

See CL 126275.

Change-Id: I66eb6a4f27f652f1b934ba702f580f6daa62a566
Reviewed-on: https://go-review.googlesource.com/127156
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/cgo/gcc.go

@@ -652,6 +652,10 @@ func (p *Package) recordTypedefs1(dtype dwarf.Type, visited map[dwarf.Type]bool)
 	visited[dtype] = true
 	switch dt := dtype.(type) {
 	case *dwarf.TypedefType:
+		if strings.HasPrefix(dt.Name, "__builtin") {
+			// Don't look inside builtin types. There be dragons.
+			return
+		}
 		if !p.typedefs[dt.Name] {
 			p.typedefs[dt.Name] = true
 			p.typedefList = append(p.typedefList, dt.Name)