arch/Config.in.arm: specify ABI for ARM64

arch/Config.in.arm: specify ABI for ARM64

There's currently only one widely supported ABI for ARM64, called lp64,
so we define BR2_GCC_TARGET_ABI to the appropriate value.

Note that there is another ABI for ARM64 being worked on, ilp32, but its
support is not fully upstream in the kernel, so we're not adding support
for it for the moment.

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>