Merge the remaining lfstack{Pack,Unpack} implemetations into one file.

unsafe.Sizeof(uintptr(0)) == 4 is a constant comparison so this branch
folds away at compile time.

Dmitry confirmed that the upper 17 bits of an address will be zero for a
user mode pointer, so there is no need to sign extend on amd64 during
unpack, so we can reuse the same implementation as all othe 64 bit
archs.

Change-Id: I99f589416d8b181ccde5364c9c2e78e4a5efc7f1
Reviewed-on: https://go-review.googlesource.com/21597
Run-TryBot: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>