Add support for ARMv8 64-bit (AArch64)

Add support for aarch64 architecture. Introduces new ROOT5 target
-- `linuxarm64`. CINT is functional, but genreflex is not capable
of generating Reflex dictionaries on aarch64. Data model, LP64,
is used by aarch64 and x86_64. As long as systems are running high
similar software you are fine with pre-generating the dictionaries
on x86_64.

The same patch will be published for ROOT developers as discussed 
with Axel.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>