pkg-cmake.mk: Set CMAKE_SYSTEM_PROCESSOR.

This is rarely needed by packages, but convenient to have when it is.

[Thomas:
  - don't define ARM_VARIANT as this name is too global, use
    CMAKE_SYSTEM_PROCESSOR_ARM_VARIANT instead.
  - don't use ifndef, but a more traditional else clause, for the
    non-ARM cases.]

Signed-off-by: Volker Krause <volker.krause@kdab.com>
Reviewed-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Reviewed-by: Romain Naour <romain.naour@openwide.fr>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>