CMake version 4 compatibility

CMake version 4 is no longer compatible with CMakefiles that set
cmake_minimum_required < 3.5. Update the required version to 3.16.0 to
match what is required by TensorFlow Lite.

Add policy override in thirdparty/CMakeLists.txt to force CMake to treat
all third party CMake files as 3.10 compatible, no matter if they
specify cmake_minimum_required and/or cmake_policy with a version
explicitly. (Currently unclear why it's required setting both a regular
variable and an environment variable for it to take effect.)

Change-Id: Ie429433fd9c8837fc3d7129c4aca41ca52560340
Signed-off-by: Jonny Svärd <jonny.svaerd@arm.com>