cmd/link: don't link sections not named .o

cmd/link: don't link sections not named .o

For many build systems, modular static analysis is most conveniently
implemented by saving analysis facts (which are analogous to export
data) in an additional section in the archive file, similar to
__PKGDEF. See golang.org/x/tools/go/analysis for an overview.

Because such sections are not object files, the linker must not
attempt to link them. This change causes the linker to skip special
sections whose name does not end with .o (and is short enough not to
be truncated).

Fixes #28429

Change-Id: I830852decf868cb017263308b114f72838032993
Reviewed-on: https://go-review.googlesource.com/c/146297
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>