runtime: fix implementation of cputicks for ppc64x

runtime: fix implementation of cputicks for ppc64x

The implementation of cputicks has been wrong for ppc64x. The
previous code sequence is for 32 bit, not 64 bit.

Change-Id: I308ae6cf9131f53a0100cd3f8ae4e16601f2d553
Reviewed-on: https://go-review.googlesource.com/129595
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Carlos Eduardo Seo <cseo@linux.vnet.ibm.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>