feat(l0dables): Use weak references for unused handlers

Instead of actually defining each unused handler as an individual
"function" with an infinite loop, make them all reference the default
handler.  This shaves 192 bytes of unnecessary bloat off the .text
section.

l0dables/lib/crt.s

@@ -181,8 +181,7 @@ DefaultHandler:
 		b .
 
 		.macro    def_irq_handler    handler_name
-		.weak \handler_name
-		\handler_name : b .
+		.weakref \handler_name, DefaultHandler
 		.endm
 
 		/*