Vendor from golang.org/x/arch (commit f185940).

Implements #19157

Updates #12840
Updates #20762
Updates #20897
Updates #20096
Updates #20766
Updates #20752
Updates #20096
Updates #19142

Change-Id: Idefb8ba2c355dc07f3b9e8dcf5f00173256a0f0f
Reviewed-on: https://go-review.googlesource.com/49530Reviewed-by: Cherry Zhang <cherryyz@google.com>