Revert "Change relocation mode to Default from PIC"

This reverts commit 90a72c1.

The bug was in the wrong length to `memcpy` for fixed array
initializers. And we apparently need PIC to build correctly on some OSs.

lib/codegen/codegen.ml

@@ -1441,7 +1441,7 @@ let generate ~target ~outname ~release ~modul
   Llvm_all_backends.initialize ();
 
   let target = Llvm_target.Target.by_triple triple in
-  let reloc_mode = Llvm_target.RelocMode.Default in
+  let reloc_mode = Llvm_target.RelocMode.PIC in
   let machine = TargetMachine.create ~triple target ~reloc_mode in
   let layout = DataLayout.as_string (TargetMachine.data_layout machine) in