[kernel] Add (wip) futex syscalls

Add the syscalls j6_futex_wait and j6_futex_wake. Currently marking this
as WIP as they need more testing.

Added to support futexes:
- vm_area and vm_space support for looking up physical address for a
  virtual address
- libj6 mutex implementation using futex system calls

src/kernel/kernel.module

@@ -57,6 +57,7 @@ kernel = module("kernel",
         "syscalls/mailbox.cpp",
         "syscalls/object.cpp",
         "syscalls/process.cpp",
+        "syscalls/futex.cpp",
         "syscalls/system.cpp",
         "syscalls/thread.cpp",
         "syscalls/vm_area.cpp",