Bug 7458: Let dfmc-llvm-linker emit (currently empty) _glue.bc files

* sources/dfmc/llvm-linker/llvm-gluefile.dylan (emit-gluefile): Emit an
  empty _glue.bc bitcode file within the build directory of built libraries.

sources/dfmc/llvm-linker/llvm-gluefile.dylan

@@ -19,7 +19,21 @@ define sideways method emit-gluefile
           debug-info? = #t,
           compilation-layer,
      #all-keys)
+  let locator
+    = build-area-output-locator(ld, base: "_glue", type: "bc");
+  let m = make(<llvm-module>,
+               name: "_glue",
+               target-triple: llvm-back-end-target-triple(back-end),
+               data-layout: llvm-back-end-data-layout(back-end));
+  back-end.llvm-builder-module := m;
+
   // FIXME
+
+  // Output LLVM bitcode
+  llvm-save-bitcode-file(m, locator);
+
+  // Retract
+  back-end.llvm-builder-module := #f;
 end;