Considerable rewriting of the parser and assembler (code generator)
but it's simpler and shorter now. The internal Addr type is gone; so
is the package that held it. Parsing of operands goes directly into
obj.Addrs now.

There is a horrible hack regarding register pairs. It uses the Class
field to store the second register since it needs _some_ place to
put it but none is provided in the API. An alternative would be nice
but this works for now.

Once again creates identical .6 and .8 files as the old assembler.

Change-Id: I8207d6dfdfdb5bbed0bd870cb34ee0fe61c2fbfd
Reviewed-on: https://go-review.googlesource.com/4062Reviewed-by: Russ Cox <rsc@golang.org>