atmel-samd: UART: allocate rx buffer in long-lived region

This is not strictly needed in order for #1056 to be resolved,
because the "make long-lived" machinery is unaware of this pointer.

However, as UARTs are assumed to be long-lived, this change is
beneficial because it moves the long-lived buffer into the upper
memory area with other long-lived objects, instead of remaining in
the low heap.

ports/atmel-samd/common-hal/busio/UART.c

@@ -132,7 +132,13 @@ void common_hal_busio_uart_construct(busio_uart_obj_t *self,
 
     if (rx && receiver_buffer_size > 0) {
         self->buffer_length = receiver_buffer_size;
-        self->buffer = (uint8_t *) gc_alloc(self->buffer_length * sizeof(uint8_t), false, false);
+        // Initially allocate the UART's buffer in the long-lived part of the
+        // heap.  UARTs are generally long-lived objects, but the "make long-
+        // lived" machinery is incapable of moving internal pointers like
+        // self->buffer, so do it manually.  (However, as long as internal
+        // pointers like this are NOT moved, allocating the buffer
+        // in the long-lived pool is not strictly necessary)
+        self->buffer = (uint8_t *) gc_alloc(self->buffer_length * sizeof(uint8_t), false, true);
         if (self->buffer == NULL) {
             common_hal_busio_uart_deinit(self);
             mp_raise_msg(&mp_type_MemoryError, "Failed to allocate RX buffer");