cmd/internal/obj: rename *.out.go to a.out.go

The old numerical names like 6.out.go are a relic from the old tools.
Easier to rename than explain.

The anames.go files were modified by go generate; no changes
beyond the explanatory comment at the top.

Change-Id: I84742c75c60e47724baa9d49a91fef1f8581f021
Reviewed-on: https://go-review.googlesource.com/12069
Run-TryBot: Rob Pike <r@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/internal/obj/arm/anames.go

-// Generated by stringer -i 5.out.go -o anames.go -p arm
+// Generated by stringer -i a.out.go -o anames.go -p arm
 // Do not edit.
 
 package arm

src/cmd/internal/obj/arm64/anames.go

-// Generated by stringer -i 7.out.go -o anames.go -p arm64
+// Generated by stringer -i a.out.go -o anames.go -p arm64
 // Do not edit.
 
 package arm64

src/cmd/internal/obj/ppc64/anames.go

-// Generated by stringer -i 9.out.go -o anames.go -p ppc64
+// Generated by stringer -i a.out.go -o anames.go -p ppc64
 // Do not edit.
 
 package ppc64

src/cmd/internal/obj/x86/anames.go

-// Generated by stringer -i 6.out.go -o anames.go -p x86
+// Generated by stringer -i a.out.go -o anames.go -p x86
 // Do not edit.
 
 package x86