Align tflite::BufferPlan properly (#2698)

In the non_persistent_buffer_planner_shim_test, a tflite::BufferPlan is created using a statically allocated buffer. A static array is allocated and the address of it is used with a reinterpret_cast. This is undefined behavior, as it could result in misaligned member accesses. This PR forces the statically allocated array to align as if it was a tflite::BufferPlan, and then uses placement new to construct it within the buffer.

BUG=none

tensorflow/lite/micro/memory_planner/non_persistent_buffer_planner_shim_test.cc

@@ -34,9 +34,10 @@ tflite::BufferPlan* CreateBufferPlan() {
  // Some targets do not support dynamic memory (i.e., no malloc or new), thus,
  // the test need to place non-transitent memories in static variables. This is
  // safe because tests are guarateed to run serially.
- static int8_t buffer_plan_buffer[tflite::SizeOfBufferPlan(kBufferCnt)];
+ alignas(tflite::BufferPlan) static int8_t
+ buffer_plan_buffer[tflite::SizeOfBufferPlan(kBufferCnt)];
  tflite::BufferPlan* buffer_plan_ptr =
- reinterpret_cast<tflite::BufferPlan*>(buffer_plan_buffer);
+ new (buffer_plan_buffer) tflite::BufferPlan();
  buffer_plan_ptr->buffer_count = kBufferCnt;
  buffer_plan_ptr->buffer_plan_entries[0].offset = kBuffer0Offset;
  buffer_plan_ptr->buffer_plan_entries[1].offset = kBuffer1Offset;