cmd/compile: allow body-less functions under -complete flag if they have a //go:linkname attribute

[mdempsky]

Currently the -complete flag (which is automatically set by cmd/go for packages without assembly source) disallows function declarations without a body. We should relax this to still allow function declarations without a body as long as they have a //go:linkname attribute.

[gopherbot]

Change https://golang.org/cl/151318 mentions this issue: cmd/compile: allow bodyless function if it is linkname'd