[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/libraries/j6/j6.module

@@ -6,6 +6,7 @@ j6 = module("j6",
     sources = [
         "init.cpp",
         "init.s",
+        "mutex.cpp",
         "protocol_ids.cpp",
         "syscalls.s.cog",
         "sysconf.cpp.cog",
@@ -17,6 +18,7 @@ j6 = module("j6",
         "j6/errors.h",
         "j6/flags.h",
         "j6/init.h",
+        "j6/mutex.hh",
         "j6/protocols.h",
         "j6/protocols/service_locator.h",
         "j6/syscalls.h.cog",