pkg-cmake.mk: Set CMAKE_SYSTEM_PROCESSOR_ARM_VARIANT for ARMv8

This is needed for correctly building some CMake-based packages which
use this variable. For example, this is needed for WebKitGTK+ 2.16.x
to build correctly when an ARMv8 target is configured.

Signed-off-by: Adrian Perez de Castro <aperez@igalia.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>