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.

3 files changed, 8 insertions, 5 deletions

diff --git a/supervisor/port.h b/supervisor/port.h
index ddb96bd52..f5b3c15d1 100644
--- a/supervisor/port.h
+++ b/supervisor/port.h
@@ -44,7 +44,7 @@ extern uint32_t _ebss;
 safe_mode_t port_init(void);
 
 // Reset the microcontroller completely.
-void reset_cpu(void);
+void reset_cpu(void) NORETURN;
 
 // Reset the microcontroller state.
 void reset_port(void);
@@ -53,7 +53,7 @@ void reset_port(void);
 void reset_board(void);
 
 // Reset to the bootloader
-void reset_to_bootloader(void);
+void reset_to_bootloader(void) NORETURN;
 
 // Get stack limit address
 uint32_t *port_stack_get_limit(void);
diff --git a/supervisor/shared/safe_mode.h b/supervisor/shared/safe_mode.h
index 7d3cd63b5..34fc3c8ae 100644
--- a/supervisor/shared/safe_mode.h
+++ b/supervisor/shared/safe_mode.h
@@ -27,6 +27,8 @@
 #ifndef MICROPY_INCLUDED_SUPERVISOR_SAFE_MODE_H
 #define MICROPY_INCLUDED_SUPERVISOR_SAFE_MODE_H
 
+#include "py/mpconfig.h"
+
 typedef enum {
   NO_SAFE_MODE = 0,
   BROWNOUT,
@@ -48,7 +50,7 @@ typedef enum {
 safe_mode_t wait_for_safe_mode_reset(void);
 
 void safe_mode_on_next_reset(safe_mode_t reason);
-void reset_into_safe_mode(safe_mode_t reason);
+void reset_into_safe_mode(safe_mode_t reason) NORETURN;
 
 void print_safe_mode_message(safe_mode_t reason);
 
diff --git a/supervisor/stub/safe_mode.c b/supervisor/stub/safe_mode.c
index 8072be2c6..a70ac6b6d 100644
--- a/supervisor/stub/safe_mode.c
+++ b/supervisor/stub/safe_mode.c
@@ -26,14 +26,15 @@
 
 #include "supervisor/shared/safe_mode.h"
 
+#include <stdlib.h>
+
 safe_mode_t wait_for_safe_mode_reset(void) {
     return NO_SAFE_MODE;
 }
 
 void reset_into_safe_mode(safe_mode_t reason) {
     (void) reason;
-    for (;;) {
-    }
+    abort();
 }
 
 void print_safe_mode_message(safe_mode_t reason) {