Kernel: Move syscall implementations out of Process.cpp

This is something I've been meaning to do for a long time, and here we
finally go. This patch moves all sys$foo functions out of Process.cpp
and into files in Kernel/Syscalls/.

It's not exactly one syscall per file (although it could be, but I got
a bit tired of the repetitive work here..)

This makes hacking on individual syscalls a lot less painful since you
don't have to rebuild nearly as much code every time. I'm also hopeful
that this makes it easier to understand individual syscalls. :^)

Kernel/Scheduler.cpp

@@ -610,6 +610,7 @@ void Scheduler::timer_tick(const RegisterState& regs)
 {
     ASSERT_INTERRUPTS_DISABLED();
     ASSERT(Processor::current().in_irq());
+
     if (Processor::current().id() > 0)
         return;
     auto current_thread = Processor::current().current_thread();