cmd/cgo: don't #include cgo_export.h in gccgo _cgo_main.c

cmd/cgo: don't #include cgo_export.h in gccgo _cgo_main.c

Instead of relying on the asm names declared in the gccgo version of
cgo_export.h, just emit a dummy symbol with the right asm name.  This
is enough to let the _cgo_main link succeed, which is all that matters
here.

Fixes #9294.

Change-Id: I803990705b6b226ed0adf17dc57b58a9f501b213
Reviewed-on: https://go-review.googlesource.com/1901
Reviewed-by: Minux Ma <minux@golang.org>