Kernel: Fix the return type for syscalls

The Process::Handler type has KResultOr<FlatPtr> as its return type.
Using a different return type with an equally-sized template parameter
sort of works but breaks once that condition is no longer true, e.g.
for KResultOr<int> on x86_64.

Ideally the syscall handlers would also take FlatPtrs as their args
so we can get rid of the reinterpret_cast for the function pointer
but I didn't quite feel like cleaning that up as well.

Kernel/Syscalls/beep.cpp

@@ -9,7 +9,7 @@
 
 namespace Kernel {
 
-KResultOr<int> Process::sys$beep()
+KResultOr<FlatPtr> Process::sys$beep()
 {
     PCSpeaker::tone_on(440);
     auto result = Thread::current()->sleep(Time::from_nanoseconds(200'000'000));