Skip TestObjdumpPowerManual if the host system is not ppc64 or ppc64le.
This test depends on using the host objdump and comparing output, which
does not work as expected if the test is run on another host.

Orignates from golang.org/x/arch/ppc64/ppc64asm commit 8e2d4898.

Fixes #17698

Change-Id: I956b0fb78c5ec33641db752d46a755008403d269
Reviewed-on: https://go-review.googlesource.com/32531Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>