Convert ODIN_OS and ODIN_ARCH to use enums rather than use strings

core/image/png/example.odin

@@ -207,7 +207,7 @@ write_image_as_ppm :: proc(filename: string, image: ^image.Image) -> (success: b
 	}
 
 	mode: int = 0
-	when ODIN_OS == "linux" || ODIN_OS == "darwin" {
+	when ODIN_OS == .Linux || ODIN_OS == .Darwin {
 		// NOTE(justasd): 644 (owner read, write; group read; others read)
 		mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH
 	}