runtime: merge 64bit lfstack impls

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>

src/runtime/lfstack_linux_ppc64x.go → src/runtime/lfstack_64bit.go

@@ -2,8 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build ppc64 ppc64le
-// +build linux
+// +build arm64 mips64 mips64le ppc64 ppc64le
 
 package runtime
 
@@ -13,6 +12,10 @@ import "unsafe"
 // TASK_SIZE_USER64 in the Linux kernel).  This has grown over time,
 // so here we allow 48 bit addresses.
 //
+// On mips64, Linux limits the user address space to 40 bits (see
+// TASK_SIZE64 in the Linux kernel).  This has grown over time,
+// so here we allow 48 bit addresses.
+//
 // In addition to the 16 bits taken from the top, we can take 3 from the
 // bottom, because node must be pointer-aligned, giving a total of 19 bits
 // of count.