Add some NORETURN attributes

[jepler]

I have (or will have, in stm32 canbus) a function where it should be impossible to reach the end, so I put in a safe-mode reset at the bottom:

int find_unused_slot(void) {
    // precondition: you already verified that a slot was available
    assert(available_slot());
    for (int i=0; i<NUM_SLOTS; i++) {
        if (slot_free(i)) {
            return i;
        }
    }
    safe_mode_reset(MICROPY_FATAL_ERROR);
}

However, the compiler still gave a diagnostic, because safe_mode_reset was not declared NORETURN.

So I started by teaching the compiler that reset_into_safe_mode never returned. This leads at least one level deeper due to reset_cpu needing to be a NORETURN function. Each port is a little different in this area. I also marked reset_to_bootloader as NORETURN.
Additional notes:

• stm32's reset_to_bootloader was not implemented, but now does a bare reset. Most stm32s are not fitted with uf2 bootloaders anyway.
• ditto cxd56
• esp32s2 did not implement reset_cpu at all. I used esp_restart(). (not tested)
• litex did not implement reset_cpu at all. I used reboot_ctrl_write. But notably this is what reset_to_bootloader already did, so one or the other must be incorrect (not tested). reboot_ctrl_write cannot be declared NORETURN, as it returns unless the special value 0xac is written), so a new unreachable forever-loop is added.
• cxd56's reset is via a boardctl() call which can't generically be declared NORETURN, so a new unreacahble "for(;;)" forever-loop is added.
• In several places, NVIC_SystemReset is redeclared with NORETURN applied. This is accepted just fine by gcc. I chose this as preferable to editing the multiple copies of CMSIS headers where it is normally declared.
• the stub safe_mode reset simply aborts. This is used in mpy-cross.

[tannewt]

Looks good to me! Thank you!

[tannewt]

Looks like iMX RT and ESP32-S2 builds are broken.

[tannewt]

Please resolve the conflict. Looks good otherwise.

[tannewt]

Thank you!