bs-platform: workaround -fno-common build failure on aarch64

Without the change -fno-common toolchain like llvm-11 fails as:

    ld: libcamlrun.a(minor_gc.o):/build/ocaml/byterun/caml/major_gc.h:67: multiple definition of
      `caml_major_ring'; libcamlrun.a(stacks.o):/build/ocaml/byterun/caml/major_gc.h:67: first defined here

pkgs/development/compilers/bs-platform/ocaml.nix

@@ -5,6 +5,13 @@ stdenv.mkDerivation rec {
   configurePhase = ''
     ./configure -prefix $out
   '';
+
+  # Workaround ocaml-4.06 limitation of duplicate definitions.
+  #   ld: libcamlrun.a(minor_gc.o):/build/ocaml/byterun/caml/major_gc.h:67: multiple definition of
+  #     `caml_major_ring'; libcamlrun.a(stacks.o):/build/ocaml/byterun/caml/major_gc.h:67: first defined here
+  # Match -fcommon workaround in ocaml-4.06 itself.
+  NIX_CFLAGS_COMPILE = "-fcommon";
+
   buildPhase = ''
     make -j9 world.opt
   '';