cmd/vendor: update vendored x/arch repo to 8a70ba74b3a1

Update vendored x/arch repo to pick up the fix of issue #33802.

This is done with the following commands:

$ cd $GOROOT/src/cmd
$ go get -d golang.org/x/arch@latest
go: finding golang.org/x/arch latest
go: downloading golang.org/x/arch v0.0.0-20190815191158-8a70ba74b3a1
go: extracting golang.org/x/arch v0.0.0-20190815191158-8a70ba74b3a1
$ go mod tidy
$ go mod vendor

Fixes #33802.

Change-Id: I0a44f1d83d6f573124cea1f099378b1c851f3feb
Reviewed-on: https://go-review.googlesource.com/c/go/+/191619


Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>