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>

src/cmd/compile/internal/gc/go.go

@@ -305,12 +305,6 @@ const (
 	Ecomplit  = 1 << 11 // type in composite literal
 )
 
-type Typedef struct {
-	Name   string
-	Etype  EType
-	Sameas EType
-}
-
 type Sig struct {
 	name   string
 	pkg    *Pkg
@@ -670,7 +664,6 @@ type Arch struct {
 	Thechar      int
 	Thestring    string
 	Thelinkarch  *obj.LinkArch
-	Typedefs     []Typedef
 	REGSP        int
 	REGCTXT      int
 	REGCALLX     int // BX