time: optimize big4

Use the same load order in big4 as in encoding/binary.BigEndian.
This order is recognized by the compiler and converted into single load.
This isn't in the hot path, but doesn't hurt readability, so lets do this.

Change-Id: Ib1240d0b278e9d667ad419fe91fa52b23d28cfc0
Reviewed-on: https://go-review.googlesource.com/130478


Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>