feat(pl/pc): add APIC timer support

src/platform/um/timer.zig

@@ -50,11 +50,13 @@ var tc: ke.TimeCounter = .{
 };
 
 pub fn init() void {
-    var act: posix.Sigaction = undefined;
+    var act = std.mem.zeroes(posix.Sigaction);
 
     act.handler.sigaction = timer_handler;
     act.flags = posix.SA.RESTART | posix.SA.SIGINFO;
 
+    act.mask = posix.sigfillset();
+
     posix.sigaction(posix.SIG.ALRM, &act, null);
 
     ke.time.register_source(&tc);