rp2/modmachine: Add mutual exclusion for machine.lightsleep().

There's no specified behaviour for what should happen if both CPUs call
`lightsleep()` together, but the latest changes could cause a permanent
hang due to a race in the timer cleanup code.  Add a flag to prevent hangs
if two threads accidentally lightsleep, at least.

This allows the new lightsleep test to pass on RPI_PICO and RPI_PICO2, and
even have much tighter time deltas.  However, the test still fails on
wireless boards where the lwIP tick wakes them up too frequently.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

ports/rp2/modmachine.c

@@ -173,6 +173,16 @@ static void mp_machine_lightsleep(size_t n_args, const mp_obj_t *args) {
     }
     #endif
 
+    #if MICROPY_PY_THREAD
+    static bool in_lightsleep;
+    if (in_lightsleep) {
+        // The other CPU is also in machine.lightsleep()
+        MICROPY_END_ATOMIC_SECTION(my_interrupts);
+        return;
+    }
+    in_lightsleep = true;
+    #endif
+
     #if MICROPY_HW_ENABLE_USBDEV
     // Only disable the USB clock if a USB host has not configured the device
     // or if going to DORMANT mode.
@@ -320,6 +330,12 @@ static void mp_machine_lightsleep(size_t n_args, const mp_obj_t *args) {
         #endif
         #endif
     }
+
+    #if MICROPY_PY_THREAD
+    // Clearing the flag here is atomic, and we know we're the ones who set it
+    // (higher up, inside the critical section)
+    in_lightsleep = false;
+    #endif
 }
 
 NORETURN static void mp_machine_deepsleep(size_t n_args, const mp_obj_t *args) {