map GNU lib extension

src/linker/lnk_config.c

@@ -119,6 +119,7 @@ global read_only struct
   { "obj",  LNK_Input_Obj },
   { "lib",  LNK_Input_Lib },
   { "rlib", LNK_Input_Lib }, // rust libs
+  { "a",    LNK_Input_Lib }, // GNU lib
   { "res",  LNK_Input_Res },
   { "rrt",  LNK_Input_RRT },
 };