reflect: unexport FFICallbackGo; use go:linkname instead

The function was always intended to be internal-only, but was exported
so that C code could call it. Now that have go:linkname for that, use it.

Change-Id: Ied5070416bb1185aa9d1d8fb81d248f90b33f5bb
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/195857
Reviewed-by: Than McIntosh <thanm@google.com>
2 files changed