arm: clean up build variables

arm: clean up build variables

This consolidates some of the different variables used for the ARM
builds. This change was prompted by the Kconfig changes but looking back
in time the CONFIG_ARM_{32,64} variables existed before Kconfig so this
should just be a generic cleanup.

Signed-off-by: Doug Goldstein <cardoe@cardoe.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- switch from ifdef X to ifeq (X,y) in one place as discussed ]