cmd: add new common architecture representation

Information about CPU architectures (e.g., name, family, byte
ordering, pointer and register size) is currently redundantly
scattered around the source tree. Instead consolidate the basic
information into a single new package cmd/internal/sys.

Also, introduce new sys.I386, sys.AMD64, etc. names for the constants
'8', '6', etc. and replace most uses of the latter. The notable
exceptions are a couple of error messages that still refer to the old
char-based toolchain names and function reltype in cmd/link.

Passes toolstash/buildall.

Change-Id: I8a6f0cbd49577ec1672a98addebc45f767e36461
Reviewed-on: https://go-review.googlesource.com/21623
Reviewed-by: Michael Hudson-Doyle <michael.hudson@canonical.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/src/cmd/compile/internal/gc/go.go b/src/cmd/compile/internal/gc/go.go
index 4cb985b..ef8b516 100644
--- a/src/cmd/compile/internal/gc/go.go
+++ b/src/cmd/compile/internal/gc/go.go
@@ -360,9 +360,8 @@
 )
 
 type Arch struct {
-	Thechar      int
-	Thestring    string
-	Thelinkarch  *obj.LinkArch
+	LinkArch *obj.LinkArch
+
 	REGSP        int
 	REGCTXT      int
 	REGCALLX     int // BX