Incorrect decision-making about 32/64-bit arm

Created by: benjaminmordaunt

CMAKE_SYSTEM_PROCESSOR sometimes reports "arm64" (or similar e.g. "arm64-v8a") instead of "aarch64" as you are expecting here. For example, this happens for GCC on macOS. This results in CMake incorrectly trying to build for 32-bit arm, flagging "-mthumb" as a nonsense flag on those systems.