cmd/cgo: fail on v, err := C.fn when fn is a builtin function

cmd/cgo: fail on v, err := C.fn when fn is a builtin function

We were already checking for _CMalloc, but in fact none of the
builtin functions support returning an error.

Fixes #67707

Change-Id: I0ee432a9f13ace472c3f36f641efc7d18eda0631
Reviewed-on: https://go-review.googlesource.com/c/go/+/589575
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>