Move the C header style architecture constants to the per arch Main
methods.

Change-Id: Ie7ff39baa275ceaa6680e7d16441ca9f0aa12597
Reviewed-on: https://go-review.googlesource.com/20722Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>