common.mk: set CFG_ARM64_core=n when COMPILE_S_KERNEL != 64

common.mk currently assumes that optee_os defaults to a 32-bit build
for the TEE core, in other words that CFG_ARM64_core is not set by
default and that only a 64-bit build requires setting CFG_ARM64_core=y.

Let's set CFG_ARM64_core=n explicitly when a 32-bit build is desired.
This will let platforms such as QEMUv8 (PLATFORM=vexpress-qemu_armv8a)
to select a 64-bit core by default in optee_os which makes more sense
since (1) build.git would not support 32-bit until parent commit
("qemu_v8: do not force COMPILE_S_KERNEL := 64"), and (2) 32-bit is
usually tested with QEMU (PLATFORM=vexpress-qemu_virt).

Signed-off-by: Jerome Forissier <jerome@forissier.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>