arch/arm: fix -mcpu default values for AArch64

We have to specify the -mcpu value, even in 64-bit mode.

For AArch64, +fp and +simd are the default, so they are totally useless.

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