arch/Timer: Make sure ARCH_TIMER_FREQ is a power of two

(avoid division and modulo, division is slow)
Fortunately, GCC will optimize divisions by powers of two to simple bitwise shifts :)

kernel/src/arch/Timer.cpp

@@ -2,8 +2,10 @@
 #include "Log.h"
 #include "arch/Serial.h"
 #include "boot/bootboot.h"
+#include <luna/TypeTraits.h>
 
-// FIXME: Storing these values as unsigned integers doesn't allow for pre-epoch times.
+// NOTE: Storing these values as unsigned integers doesn't allow for pre-epoch times.
+// We are in 2023 anyway, not sure why anybody would want to set their computer's time to 1945.
 static u64 timer_ticks = 0;
 static u64 boot_timestamp;
 
@@ -134,8 +136,9 @@ namespace Timer
     }
 }
 
+static_assert(IsPowerOfTwo<usize, ARCH_TIMER_FREQ>, "ARCH_TIMER_FREQ must be a power of two");
+
 bool should_invoke_scheduler()
 {
-    // FIXME: Modulo is SLOW. We're calling this every tick.
     return (timer_ticks % ARCH_TIMER_FREQ) == 0;
 }

kernel/src/arch/x86_64/Timer.h

@@ -1,4 +1,4 @@
 #pragma once
 #include <luna/Types.h>
 
-const usize ARCH_TIMER_FREQ = 5;
+const usize ARCH_TIMER_FREQ = 4;