cmd/go: implement gccgo support for content-based staleness

[rsc]

CL 73214 does not contain support for gccgo-based builds
and instead will cause cmd/go to fatal out if used with gccgo.
(The CL is a bit too large already.)

This issue is to make gccgo work again. The main work items are:

1. Figure out where to store build IDs in package archives and do so.

2. Write support for buildActionID/linkActionID/linkSharedActionID to record the relevant build configuration input for the gccgo toolchain.

[rsc]

Introduced in CL 73214:

$ git grep -n issue/22472
misc/cgo/testshared/shared_test.go:601:	t.Skip("golang.org/issue/22472")
src/cmd/go/go_test.go:735:		t.Skip("golang.org/issue/22472")
src/cmd/go/go_test.go:2655:	t.Skip("golang.org/issue/22472")
$

[gopherbot]

Change https://golang.org/cl/85555 mentions this issue: cmd/go: add support for build IDs with gccgo

[rsc]

With apologies for not doing this myself, I'm going to assign this to you @ianlancetaylor. Your CL is the lion's share I believe.

[ianlancetaylor]

As far as I can tell, this is done. We can open new issues for any problems that arise.

[tklauser]

TestIssue7573 and the gccgo specific part of TestBuildComplex in src/cmd/go/go_test.go are still skipped with this issue as a log message:

% git grep issue/22472
cmd/go/go_test.go:802:          t.Skip("golang.org/issue/22472")
cmd/go/go_test.go:3087: t.Skip("golang.org/issue/22472")

Should these skips be removed?

[gopherbot]

Change https://golang.org/cl/120375 mentions this issue: cmd/go: re-enable a couple of tests of gccgo

[ianlancetaylor]

Yes. Sent https://golang.org/cl/120375. Thanks.

[gopherbot]

Change https://golang.org/cl/120395 mentions this issue: cmd/go: re-enable a couple of tests of gccgo