Fix alloc.c as per #386, define INVALID_ constants and rename malloc to alloc as per #325

Some of the INVALID_ are likely erroneously placed, due to lack of documentation

sym_bss.txt

@@ -1,5 +1,5 @@
 	.include "src/main.o"
-	.include "src/malloc.o"
+	.include "src/alloc.o"
 	.include "src/dma3_manager.o"
 	.include "src/gpu_regs.o"
 	.include "src/bg.o"