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 <b27445fe6fb259c37b8e0a4ce56e22e2413754bf@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Carlos Eduardo Seo <66998daf164cd5aaa90ed173e5a4d099ecd36f2c@linux.vnet.ibm.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

CAFxX/go,golang/go,josharian/go,cloudflare/go,josharian/go,josharian/go,CAFxX/go,josharian/go,cloudflare/go,cloudflare/go,cloudflare/go,akutz/go,nawawi/go,josharian/go,akutz/go,golang/go,CAFxX/go,nawawi/go,josharian/go,akutz/go,nawawi/go,akutz/go,cloudflare/go,golang/go,golang/go,CAFxX/go,CAFxX/go,josharian/go,akutz/go,akutz/go,akutz/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,golang/go,nawawi/go,nawawi/go,CAFxX/go,cloudflare/go,golang/go,golang/go,CAFxX/go,cloudflare/go,akutz/go,nawawi/go,josharian/go,cloudflare/go