cmd/link: provide a way to always use external linking if cgo is used

[mwhudson]

This came up as a potential solution for #43505 but I've wanted it for other reasons too (mostly because of other tools like patchelf that rely on implementation details of more common linkers): I would like a way to always use external linking if cgo is used, or to put it another way, never use internal linking to generate a dynamically linked binary. -linkmode and GO_EXTLINK_ENABLED control aspects of this behaviour but can't be used to get what I want.

[toothrot]

/cc @cherrymui @rsc @mdempsky @ianlancetaylor

[cherrymui]

Could you explain why -linkmode=external doesn't do what you want? It will use external linking even if cgo is not used, but that doesn't seem a big problem.

[mwhudson]

If you always external link, you always get a dynamic executable. I wouldn't mind that, but I thinks some other people might.