[dev.power64] runtime: define page size on power64le systems to be 64k

Fixes issue 8495.

CL 128260043 updated the definition of syscall.GetPageSize to report 64k for power64 and power64le. This CL cleans up the last place where the page size was defined as 4k.

LGTM=minux
R=rsc, minux
CC=golang-codereviews
https://codereview.appspot.com/123520043