[kernel] Add userspace threading

Implement the syscalls necessary for threads to create other threads in
their same process. This involved rearranging a number of syscalls, as
well as implementing object_wait and a basic implementation of a
process' list of handles.

src/kernel/syscalls/system.cpp

@@ -0,0 +1,33 @@
+#include "j6/errors.h"
+#include "j6/types.h"
+
+#include "log.h"
+#include "scheduler.h"
+
+namespace syscalls {
+
+j6_status_t
+system_log(const char *message)
+{
+	if (message == nullptr) {
+		return j6_err_invalid_arg;
+	}
+
+	auto &s = scheduler::get();
+	TCB *tcb = s.current();
+	thread *th = thread::from_tcb(tcb);
+	log::info(logs::syscall, "Message[%llx]: %s", th->koid(), message);
+	return j6_status_ok;
+}
+
+j6_status_t
+system_noop()
+{
+	auto &s = scheduler::get();
+	TCB *tcb = s.current();
+	thread *th = thread::from_tcb(tcb);
+	log::debug(logs::syscall, "Thread %llx called noop syscall.", th->koid());
+	return j6_status_ok;
+}
+
+} // namespace syscalls