cmd/go: drop runtime from plugins

[rasky]

It looks like plugins contain a copy of the runtime. I guess that copy is unused, and could probably be dropped, reducing binary size.

[rasky]

/cc @crawshaw

[crawshaw]

Yup, I had a TODO about this I lost somewhere in the CL patchsets.

It should be straightforward, it's already possible to build -linkshared binaries without a runtime. I expect most of the work is careful argument construction in cmd/go.

[rsc]

@crawshaw, still want to do this for Go 1.8?

[crawshaw]

Pushing to 1.9, so I can spend what cycles I have on bugs.