test/codegen: accept ppc64x as alias for ppc64le and ppc64 arches

test/codegen: accept ppc64x as alias for ppc64le and ppc64 arches

This helps simplify the noise when adding ppc codegen tests. ppc64x
is used in other places to indicate something which runs on either
endian.

This helps cleanup existing codegen tests which are mostly
identical between endian variants.

condmove tests are converted as an example.

Change-Id: I2b2d98a9a1859015f62db38d62d9d5d7593435b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/462895
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Paul Murphy <murp@ibm.com>