arch/Config.in.arm: prepare addition of 64 bits cores

arch/Config.in.arm: prepare addition of 64 bits cores

Until now the "Target Architecture Variant" choice was not visible on
AArch64. In order to prepare the addition of the 64 bits core to this
choice, this commit adds a "depends on !BR2_ARCH_IS_64" dependency to
all currently supported cores (that are 32 bits only).

Following this commit, the "Target Architecture Variant" choice appears
on AArch64, but is for now empty.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>