Macro for implementing vectored interrupts

[romancardenas]

I developed a macro by example to ease the definition of vectored interrupt handlers (see rust-embedded/riscv#158). The macro accepts one pattern with four parameters:

• INTERRUPT: interrupt callback function to be defined. Usually, it is set to MachineExternal. However, I left this parameter open so you can easily implement vector callbacks for other interrupt types.
• CLAIM: function to claim/deduce the interrupt source number. Its signature must be fn() -> u16.
• EXTI_TABLE: path to the table of external interrupt callbacks. This must be a static array of callbacks for the different interrupt sources. Its signature must be static [unsafe extern "C" fn(); N_EXTIS], where N_EXTIS corresponds to the number of different interrupt sources of the target. Note that interrupt number 0 is reserved for "no interrupt". Thus, the callback for interrupt number i must be in the i - 1 index of the array.
• COMPLETE: function to mark a pending interrupt as complete. Its signature must be fn(u16).

As a simple example, you could do something like this:

// Just a silly example, don't pay attention to the implementation
fn claim() -> u16 {0}
static EXTI: [unsafe extern "C" fn(); 1] = [GPIO0]; 
fn complete(_a: u16) {}

vectored_interrupt!(MachineExternal, claim, EXTI, complete);  // MachineExternal is now vectored
vectored_interrupt!(SupervisorExternal, claim, EXTI, complete);  // same for SupervisorExternal

PACs can provide a feature for implementing this automatically for the users.