cmd/compile: eliminate arch-specific typedefs

cmd/compile: eliminate arch-specific typedefs

Arch backends already provide us Widthint and Widthptr, which is ample
information to figure out how to define the universal "int", "uint",
and "uintptr" types.  No need for providing a generic typedef
mechanism beyond that.

Change-Id: I35c0c17a67c80605a9208b93d77d6960b2cbb17d
Reviewed-on: https://go-review.googlesource.com/20153
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>