The new //export comment marks a Go function as callable from
C.  The syntax is "//export NAME" where NAME is the name of
the function as seen from C.  If such a comment is seen, cgo
will generate two new files: _cgo_export.h and _cgo_export.c.
The _cgo_export.h file provides declarations which C code may
use to call Go functions.  The _cgo_export.c file contains
wrappers, and is to be compiled with gcc.

The changes to Make.pkg support using this from a Go Makefile,
though it could probably be more convenient.

R=rsc
CC=golang-dev
https://golang.org/cl/853042