[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.

modules.yaml

@@ -50,6 +50,8 @@ modules:
             - src/kernel/syscall.s
             - src/kernel/syscalls/object.cpp
             - src/kernel/syscalls/process.cpp
+            - src/kernel/syscalls/system.cpp
+            - src/kernel/syscalls/thread.cpp
             - src/kernel/task.s
             - src/kernel/crtn.s