Add more memory barriers to avoid code re-ordering issues with DMA (#155)

* Add more memory barriers to avoid code re-ordering issues with DMA

* Comment typos

* Fix Wstrict-prototype on __compiler_memory_barrier

* Remove now-redundant __compiler_barrier macro from hardware_flash

Co-authored-by: Luke Wren <wren6991@gmail.com>

src/rp2_common/hardware_dma/include/hardware/dma.h

@@ -540,6 +540,8 @@ inline static bool dma_channel_is_busy(uint channel) {
  */
 inline static void dma_channel_wait_for_finish_blocking(uint channel) {
     while (dma_channel_is_busy(channel)) tight_loop_contents();
+    // stop the compiler hoisting a non volatile buffer access above the DMA completion.
+    __compiler_memory_barrier();
 }
 
 /*! \brief Enable the DMA sniffing targeting the specified channel