buildroot

arch/arm: armv8 is really armv8a

For armv8, there are different profiles: A, M and R, like there is for
armv7.

So, rename our internal symbol to mirror what we do for armv7.

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