cmd/go: add 'unix' build tag but not *_unix.go file support

[rsc]

In #20322 we accepted a proposal to recognize unix both as a file suffix and a build tag.
@ianlancetaylor implemented it but found that there were many collisions with files named *_unix.go that would break builds.

It sounds like we should revisit this and add just the build tag, leaving unix unrecognized in file names.
This would match what we already do with tags like gc, gccgo, cgo, and so on.

So that's the proposal: add unix only as a build tag.

See #20322 (comment) and the following comments for more details.
(Opened a new issue so people don't have to read the entire conversation from before.)

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

[ianlancetaylor]

CC @robpike

[gopherbot]

Change https://go.dev/cl/389935 mentions this issue: all: use new "unix" build tag where appropriate

[gopherbot]

Change https://go.dev/cl/389934 mentions this issue: go/build: recognize "unix" build tag

[rsc]

Based on the discussion above, this proposal seems like a likely accept.
— rsc for the proposal review group

[rsc]

No change in consensus, so accepted. 🎉
This issue now tracks the work of implementing the proposal.
— rsc for the proposal review group