Kernel: Rewrite whole scheduler

Current context saving was very hacky and dependant on compiler
behaviour that was not consistent. Now we always use iret for
context saving. This makes everything more clean.

kernel/kernel/Storage/ATA/AHCI/Device.cpp

@@ -168,7 +168,7 @@ namespace Kernel
 		//        This doesn't allow scheduler to go properly idle.
 		while (SystemTimer::get().ms_since_boot() < start_time + s_ata_timeout)
 		{
-			Scheduler::get().reschedule();
+			Scheduler::get().yield();
 			if (!(m_port->ci & (1 << command_slot)))
 				return {};
 		}