LibC: Run constructors on process startup.

Cooperate with the compiler to generate and execute the _init_array list
of constructor functions on userspace program statup. This took two days
to get working, my goodness. :^)

LibC/crtn.S

@@ -0,0 +1,7 @@
+.section .init
+    pop %ebp
+    ret
+
+.section .fini
+    pop %ebp
+    ret