cmd/cgo: for gccgo add #define to cgo_export.h for expected name

cmd/cgo: for gccgo add #define to cgo_export.h for expected name

For gccgo we rename exported functions so that the compiler
will make them visible.  This CL adds a #define so that C
functions that #include "cgo_export.h" can use the expected
names of the function.

The test for this is the existing issue6833 test in
misc/cgo/test.  Without this CL it fails when using
-compiler=gccgo.

LGTM=minux.ma, rsc
R=golang-codereviews, gobot, rsc, minux.ma
CC=golang-codereviews
https://golang.org/cl/91830046