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/libl4/src/arm/syscalls.S

@@ -197,6 +197,18 @@ BEGIN_PROC(l4_space_control)
 	ldmfd	sp!, {pc}	@ Restore original lr and return.
 END_PROC(l4_space_control)
 
+/*
+ * Locks/unlocks a userspace mutex.
+ * @r0 = mutex virtual address, @r1 = mutex operation code
+ */
+BEGIN_PROC(l4_mutex_control)
+	stmfd	sp!, {lr}
+	ldr	r12, =__l4_mutex_control
+	mov	lr, pc
+	ldr	pc, [r12]
+	ldmfd	sp!, {pc}	@ Restore original lr and return.
+END_PROC(l4_mutex_control)
+
 /*
  * Sets registers of a thread and its pager.
  * @r0 = ptr to exregs_data structure, @r1 = tid of thread.