This libgo patch changes the cgo command to use runtime/cgo.Incomplete
instead of //go:notinheap, and to define the new type in the
runtime/cgo package.  This ports https://go.dev/cl/421879 to libgo.
This is a quick port to update libgo to work with the version of cgo
in gc mainline.  A more complete port will follow, changing the gc
version of cmd/cgo to choose an approach based on feature testing the
gccgo in use.  Bootstrapped and tested on x86_64-pc-linux-gnu.
Committed to mainline.

Ian