Define heap in linker and place other sections correctly

os/alloc.c

@@ -1,14 +1,16 @@
 #include "alloc.h"
 #include "stdint.h"
 
-void* *first_mem_avail = (void*) 0x20000000;
+HeapArena heap_arena;
+
+extern void * _heap_top;
 
 void init_allocator() {
-    *first_mem_avail = (void*) 0x20000000 +sizeof(void*);
+    heap_arena.wilderness = &_heap_top;
 }
 
 void* malloc(size_t size) {
-    void* block = *first_mem_avail;
-    *first_mem_avail += size;
+    void* block = heap_arena.wilderness;
+    heap_arena.wilderness += size;
     return block;
 }