misc/cgo/test/backdoor: add gccgo version of backdoor function
misc/cgo/test/backdoor: add gccgo version of backdoor function

For the gc compiler the Go function Issue7695 is defined in
runtime.c, but there is no way to do that for gccgo, because
there is no way to get the correct pkgpath.  The test is not
important for gccgo in any case.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/93870044

pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go