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>