Vendor from golang.org/x/arch repo.  Pull in commits that update
arm64, arm, ppc64 and x86 directories (the latest commit 9111c30).

Change-Id: I2b4b3ea9662e69bcf0eeee9c6aba0118175524df
Reviewed-on: https://go-review.googlesource.com/107695
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>