cmd/link/internal/ld: rename pobj.go to main.go

The only thing pobj contains is the Ldmain function.

Updates #16818

Change-Id: Id114bdb264cb5ea2f372eb2166201f1f8eb99445
Reviewed-on: https://go-review.googlesource.com/27472
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>