misc/cgo/test: get rid of the backdoor package

For Go 1.5, we can use go:linkname rather than assembly thunk for gc.
Gccgo already has support for //extern.

Change-Id: I5505aa247dd5b555112f7261ed2f192c81cf0bdf
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/1888
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>