pru: Compile the irq_poll() function with -O2 optimization

Compile the PRU binary with -Os optimization, but request that the
timer dispatch code be compiled with -O2 optimization.  This improves
the performance of timers slightly.

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

src/pru/main.c

@@ -89,7 +89,7 @@ _irq_poll(void)
     }
     CT_INTC.SECR0 = (1 << IEP_EVENT) | (1 << KICK_PRU1_EVENT);
 }
-void
+void __attribute__((optimize("O2")))
 irq_poll(void)
 {
     if (read_r31() & (1 << (WAKE_PRU1_IRQ + R31_IRQ_OFFSET)))