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/33803
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

diff --git a/src/cmd/internal/obj/obj.go b/src/cmd/internal/obj/line.go
similarity index 100%
rename from src/cmd/internal/obj/obj.go
rename to src/cmd/internal/obj/line.go