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