cmd/internal/obj: rename obj.go to line.go

This file is entirely about the implementation of LineHist, and I can
never remember which generic filename in cmd/internal/obj has it.
Rename to line.go to match the already existing line_test.go.

Change-Id: Id01f3339dc550c9759569d5610d808b17bca44d0
Reviewed-on: https://go-review.googlesource.com/33803Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>