The patch adds ARMv8 64-bit (AArch64) support to ROOT6 CMake build
files. The target is named linuxarm64 (same as in legacy build system)

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>