added /RAD_WRITE_TEMP_FILES

When enabled linker writes image and debug info to temporary files
and renames them after all writes are done.

src/linker/lnk.h

@@ -224,6 +224,7 @@ typedef struct
 typedef struct
 {
   String8 path;
+  String8 temp_path;
   String8 data;
 } LNK_WriteThreadContext;