arch/arm: add blind options to know the ARM architecture

In preparation to the removal of BR2_GCC_TARGET_ARCH for ARM, this
commit introduces a number of blind options for each ARM architecture,
so that packages/toolchains that had dependencies using
BR2_GCC_TARGET_ARCH can continue to express their dependencies. It can
also be used to simplify package dependencies that were using the
individual ARM core options.

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