trace: add workaround for SystemTap PR13296

SystemTap sdt.h sometimes results in compiled probes without sufficient
information to extract arguments.  This can be solved in a slightly
hacky way by encouraging the compiler to place arguments into registers.

This patch fixes the apic_reset_irq_delivered() trace event on Fedora 20
with gcc-4.8.2-7.fc20 and systemtap-sdt-devel-2.4-2.fc20 on x86_64.
Signed-off-by: NFrank Ch. Eigler <fche@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

hw/intc/apic_common.c

@@ -117,7 +117,12 @@ void apic_report_irq_delivered(int delivered)
 
 void apic_reset_irq_delivered(void)
 {
-    trace_apic_reset_irq_delivered(apic_irq_delivered);
+    /* Copy this into a local variable to encourage gcc to emit a plain
+     * register for a sys/sdt.h marker.  For details on this workaround, see:
+     * https://sourceware.org/bugzilla/show_bug.cgi?id=13296
+     */
+    volatile int a_i_d = apic_irq_delivered;
+    trace_apic_reset_irq_delivered(a_i_d);
 
     apic_irq_delivered = 0;
 }