Fix for GCC generating bad code for DMAChannel TCD access when it can't prove alignment.

teensy3/DMAChannel.h

 
 class DMABaseClass {
 public:
-	typedef struct __attribute__((packed)) {
+	typedef struct __attribute__((packed, aligned(4))) {
 		volatile const void * volatile SADDR;
 		int16_t SOFF;
 		union { uint16_t ATTR;
 
 class DMABaseClass {
 public:
-	typedef struct __attribute__((packed)) {
+	typedef struct __attribute__((packed, aligned(4))) {
 		volatile const void * volatile SAR;
 		volatile void * volatile       DAR;
 		volatile uint32_t              DSR_BCR;