arm_mode enum

Represent state of an ARM core. Most numbers match the five low bits of the *PSR registers on "classic ARM" processors, which build on the ARMv4 processor modes and register set. ARM_MODE_ANY is a magic value, often used as a wildcard. Only the microcontroller cores (ARMv6-M, ARMv7-M) support ARM_MODE_THREAD, ARM_MODE_USER_THREAD, and ARM_MODE_HANDLER. Those are the only modes they support.