The patch adds supports for ARMv8 64-bit (AArch64). This is done
by introducing a new target -- linuxarm64 -- to ROOT6 build scripts
and configuration.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Tested-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Signed-off-by: Danilo Piparo <danilo.piparo@cern.ch>