QuRT: workaround for __sync_bool_compare_and_swap

The Hexagon compiler version does not support __sync_bool_compare_and_swap.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/drivers/device/ringbuffer.cpp

@@ -235,7 +235,20 @@ RingBuffer::force(double val)
 }
 
 // FIXME - clang crashes on this get() call
-#ifndef __PX4_QURT
+#ifdef __PX4_QURT
+#define __PX4_SBCAP my_sync_bool_compare_and_swap
+static bool my_sync_bool_compare_and_swap(volatile unsigned *a, unsigned b, unsigned c)
+{
+	if (*a == b) {
+		*a = c;
+		return true;
+	}
+	return false;
+}
+
+#else
+#define __PX4_SBCAP __sync_bool_compare_and_swap
+#endif
 bool
 RingBuffer::get(void *val, size_t val_size) 
 {
@@ -258,14 +271,13 @@ RingBuffer::get(void *val, size_t val_size)
 				memcpy(val, &_buf[candidate * _item_size], val_size);
 
 			/* if the tail pointer didn't change, we got our item */
-		} while (!__sync_bool_compare_and_swap(&_tail, candidate, next));
+		} while (!__PX4_SBCAP(&_tail, candidate, next));
 
 		return true;
 	} else {
 		return false;
 	}
 }
-#endif
 
 bool
 RingBuffer::get(int8_t &val)