Merge all the 64bit lfstack impls into one file, adjust build tags to
match.

Merge all the comments on the various lfstack implementations for
posterity.

lfstack_amd64.go can probably be merged, but it is slightly different so
that will happen in a followup.

Change-Id: I5362d5e127daa81c9cb9d4fa8a0cc5c5e5c2707c
Reviewed-on: https://go-review.googlesource.com/21591
Run-TryBot: Dave Cheney <dave@cheney.net>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>