Mutex test progress

- Mutex test added. Forked tasks demonstrate produce/consumer using a
  shared mmap'ed page.
- Added l4lib assembler syscall
- Added forgotten SWI to mutex control offset in syscall page.
- Added mutex head initialization
- Contended child successfully sleeps in a waitqueue.

Issues:
- Somehow the child's produced page buffer is altered at about [4020] offset.
  Parent fails to validate buffer therefore.
- Need to add syncing to test so that parent does not unlock and lock again
  before child has a chance to lock buffer and produce.

tasks/test0/include/tests.h

@@ -23,5 +23,6 @@ int dirtest(void);
 int fileio(void);
 int clonetest(void);
 int exectest(void);
+int user_mutex_test(void);
 
 #endif /* __TEST0_TESTS_H__ */