HRT: Create new separate call for atomic HRT elapsed time calculation

This call rarely needs to be truly atomic and the involved CPU overhead in making it atomic was unnecessary and introduces a lot of IRQ jitter with no value-add. The call has been moved to be non-atomic and the codebase will be inspected and changed in follow-up commits for the few instances where it is truly needed.

src/drivers/stm32/drv_hrt.c

@@ -720,6 +720,17 @@ abstime_to_ts(struct timespec *ts, hrt_abstime abstime)
  */
 hrt_abstime
 hrt_elapsed_time(const volatile hrt_abstime *then)
+{
+	hrt_abstime delta = hrt_absolute_time() - *then;
+
+	return delta;
+}
+
+/**
+ * Compare a time value with the current time as atomic operation
+ */
+hrt_abstime
+hrt_elapsed_time_atomic(const volatile hrt_abstime *then)
 {
 	irqstate_t flags = px4_enter_critical_section();