contrib: toolchain.cmake: enforce variables for android
CMAKE_SYSTEM_PROCESSOR must match specific rules for Android, defined in
the CMake files: Modules/Platform/Android-Determine.cmake. Instead, pass
down the ANDROID_ABI variable to the CMAKE_ANDROID_ARCH_ABI which will be
used to set the correct environment, including the processor variable.
Also enforce CMAKE_SYSTEM_NAME to have CMAKE_CROSSCOMPILE = true and
CMAKE_ANDROID_ARM_NEON with armeabi-v7a for correct neon support and
enable switch.
Signed-off-by: Thomas Guillem <thomas@gllm.fr>