Previously, lfstack assumed Linux limited user space addresses
to 43 bits on Power64 based on a paper from 2001.  It turns
out the limit is now 46 bits, so lfstack was truncating
pointers.

Raise the limit to 48 bits (for some future proofing and to
make it match amd64) and add a self-test that will fail in a
useful way if ever unpack(pack(x)) != x.

With this change, dev.cc passes all.bash on power64le.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/174430043