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.

src/arch/arm/syscall.S

@@ -30,5 +30,6 @@ BEGIN_PROC(arm_system_calls)
 	swi	0x14		@ kread			/* 0x28 */
 	swi	0x14		@ kmem_control		/* 0x2C */
 	swi	0x14		@ time			/* 0x30 */
+	swi	0x14		@ mutex_control		/* 0x34 */
 END_PROC(arm_system_calls)