This adds timer functionality to sim65.

It provides access to a handful of 64-bit counters that count different things:
- clock cycles
- instructions
- number of IRQ processed
- number of NMIs processed
- nanoseconds since 1-1-1970.

This in not ready yet to be pushed as a merge request into the upstream CC65
repository. What's lacking:

- documentation
- tests

And to be discussed:

- do we agree on this implementation direction and interface in principe?
- can I include inttypes.h for printing a 64-bit unsigned value?
- will clock_gettime() work on a Windows build?

src/sim65/error.c

@@ -36,9 +36,10 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdarg.h>
+#include <inttypes.h>
 
 #include "error.h"
-
+#include "peripherals.h"
 
 
 /*****************************************************************************/
@@ -50,9 +51,6 @@
 /* flag to print cycles at program termination */
 int PrintCycles = 0;
 
-/* cycles are counted by main.c */
-extern unsigned long long TotalCycles;
-
 
 
 /*****************************************************************************/
@@ -120,7 +118,7 @@ void SimExit (int Code)
 /* Exit the simulation with an exit code */
 {
     if (PrintCycles) {
-        fprintf (stdout, "%llu cycles\n", TotalCycles);
+        fprintf (stdout, PRIu64 " cycles\n", PRegs.counter_clock_cycles);
     }
     exit (Code);
 }