__get_current_exception() function

Get the current exception level on this core On Cortex-M this is the exception number defined in the architecture reference, which is equal to VTABLE_FIRST_IRQ + irq num if inside an interrupt handler. (VTABLE_FIRST_IRQ is defined in platform_defs.h). On Hazard3, this function returns VTABLE_FIRST_IRQ + irq num if inside of an external IRQ handler (or a fault from such a handler), and 0 otherwise, generally aligning with the Cortex-M values.