sam3x8e: Align the TC0_Handler timer irq handler

The code alignment of the TC0_Handler function seems to noticeably
impact performance benchmarks.  Set the function alignment to 16 bytes
to improve testing consistency.  An alignment of 16 doesn't
necessarily improve performance, but it seems to improve testing
consistency on code changes unrelated to timer dispatch.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/sam3x8e/timer.c

@@ -54,7 +54,7 @@ timer_shutdown(void)
 DECL_SHUTDOWN(timer_shutdown);
 
 // IRQ handler
-void __visible
+void __visible __aligned(16) // aligning helps stabilize perf benchmarks
 TC0_Handler(void)
 {
     irq_disable();