Add some NORETURN attributes

I have 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
    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.

1 files changed, 8 insertions, 0 deletions

diff --git a/ports/litex/supervisor/port.c b/ports/litex/supervisor/port.c
index 3125072e6..e55eb49f0 100644
--- a/ports/litex/supervisor/port.c
+++ b/ports/litex/supervisor/port.c
@@ -85,9 +85,17 @@ void reset_port(void) {
 
 void reset_to_bootloader(void) {
     reboot_ctrl_write(0xac);
+    for(;;) {}
 }
 
 void reset_cpu(void) {
+    // "You can reset Fomu by writing a special value to the CSR_REBOOT_CTRL
+    // register at 0xe0006000L. All writes to this register must start with
+    // 0xac, to ensure random values aren’t written. We can reboot Fomu by
+    // simply writing this value" --
+    //     https://workshop.fomu.im/en/latest/riscv.html
+    reboot_ctrl_write(0xac);
+    for(;;) {}
 }
 
 supervisor_allocation* port_fixed_stack(void) {