Fix -target:js_wasm32 for core:runtime

core/runtime/default_allocators_nil.odin

@@ -0,0 +1,17 @@
+//+build freestanding, js
+package runtime
+
+// mem.nil_allocator reimplementation
+
+default_allocator_proc :: proc(allocator_data: rawptr, mode: Allocator_Mode,
+                               size, alignment: int,
+                               old_memory: rawptr, old_size: int, loc := #caller_location) -> ([]byte, Allocator_Error) {
+	return nil, .None;
+}
+
+default_allocator :: proc() -> Allocator {
+	return Allocator{
+		procedure = default_allocator_proc,
+		data = nil,
+	};
+}