SITL: implement cli()/sei() properly for desktop build

this disables simulated hardware interrupts during critical sections

libraries/Desktop/include/avr/interrupt.h

@@ -6,7 +6,9 @@
 #define ISR(vector,...) void vector(void);	\
 void vector(void)
 
-#define cli()
-#define sei()
+extern "C" {
+void cli(void);
+void sei(void);
+}
 
 #endif

libraries/Desktop/support/Arduino.cpp

@@ -5,6 +5,7 @@
 #include "avr/pgmspace.h"
 #include <BetterStream.h>
 #include <sys/time.h>
+#include <signal.h>
 #include "desktop.h"
 
 extern "C" {
@@ -14,6 +15,24 @@ volatile uint8_t __iomem[1024];
 unsigned __brkval = 0x2000;
 unsigned __bss_end = 0x1000;
 
+// disable interrupts
+void cli(void)
+{
+	sigset_t set;
+        sigemptyset(&set);
+        sigaddset(&set, SIGALRM);
+        sigprocmask(SIG_BLOCK,&set,NULL);
+}
+
+// enable interrupts
+void sei(void)
+{
+	sigset_t set;
+        sigemptyset(&set);
+        sigaddset(&set, SIGALRM);
+        sigprocmask(SIG_UNBLOCK,&set,NULL);
+}
+
 void pinMode(uint8_t pin, uint8_t mode)
 {