cmd/pack: treat compiler's -linkobj output as "compiler object"

Treat the compiler's -linkobj output as "compiler object, which
means "pack c" will "see through" the file and add individual
entry to the new archive, instead of the object as a whole.

This is somewhat peculiar. But Go 1.15's cmd/pack does this,
although seemingly accidental. We just do the same. FWIW, it
does make things more consistent with/without -linkobj flag.

Fixes #43271.

Change-Id: I6b2d99256db7ebf0fa430f85afa7464e334f6bcb
Reviewed-on: https://go-review.googlesource.com/c/go/+/279483


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>