The current implementation for ppc64{,le} only supports linux (through
HWCaps read from /proc/self/auxv) and fails to build e.g. on
aix/ppc64.

Change-Id: I5e1efe75c781fd08bc7bbe797df88295726ba62b
Reviewed-on: https://go-review.googlesource.com/c/162798
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>