These assemblers produce byte-for-byte identical output
to the ones written in C.

They are primarily a proof that cmd/internal/obj can be used
standalone to produce working object files. (The use via objwriter
starts by deserializing an already-constructed internal representation,
so objwriter does not exercise the code in cmd/internal/obj that
creates such a representation from scratch.)

Change-Id: I1793d8d010046cfb9d8b4d2d4469e7f47a3d3ac7
Reviewed-on: https://go-review.googlesource.com/3143Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>