cmd/cgo: remove //extern for check functions in gccgo Go prologue

The //extern comments are incorrect and cause undefined symbol
errorswhen building cgo code with -compiler=gccgo. The code is already
designed to use weak references, and that support relies on the cgo
check functions being treated as local functions.

Change-Id: Ib38a640cc4ce6eba74cfbf41ba7147ec88769ec0
Reviewed-on: https://go-review.googlesource.com/23014
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/cgo/out.go

@@ -1360,10 +1360,8 @@ func _cgoCheckResult(interface{})
 `
 
 const gccgoGoProlog = `
-//extern runtime.cgoCheckPointer
 func _cgoCheckPointer(interface{}, ...interface{}) interface{}
 
-//extern runtime.cgoCheckResult
 func _cgoCheckResult(interface{})
 `