[dev.cc] cmd/objwriter: implement using cmd/internal/obj

[dev.cc] cmd/objwriter: implement using cmd/internal/obj

New code but nothing interesting.
It's nearly all parsing code for the format written by liblink.
The interesting part is the call to obj.Writeobjdirect, which
is the Go translation of the C liblink writeobjdirect function.

Change-Id: I2e9e755e7a3c999302e2ef2c7475c0af9c5acdd2
Reviewed-on: https://go-review.googlesource.com/3047
Reviewed-by: Ian Lance Taylor <iant@golang.org>