Fix clock source unit when building on macOS

The current implementation of 'semu_timer_clocksource()' returns
the clock source in nanoseconds, but it should return in seconds
for correct system integration.

Adjust the function on macOS to divide the return value by 1e9,
ensuring the clock source is provided in seconds.

utils.c

@@ -48,7 +48,8 @@ static uint64_t semu_timer_clocksource(uint64_t freq)
     static mach_timebase_info_data_t t;
     if (t.denom == 0)
         (void) mach_timebase_info(&t);
-    return mult_frac(mach_absolute_time() * freq, t.numer, t.denom);
+    return mult_frac(mult_frac(mach_absolute_time(), freq, 1e9), t.numer,
+                     t.denom);
 #else
     return time(0) * freq;
 #endif