llvmPackages_12: Create subdirectories for the last two packages

For consistency. Now all packages will have their own subdirectory
(continuation of 781e69d19c4).

pkgs/development/compilers/llvm/12/default.nix

···
     # doesn’t support like LLVM. Probably we should move to some other
     # file.
 
-    bintools = callPackage ./bintools.nix {};
+    bintools = callPackage ./bintools {};
 
     lldClang = wrapCCWith rec {
       cc = tools.clang-unwrapped;
···
         libunwind = libraries.libunwind;
       }));
 
-    openmp = callPackage ./openmp.nix { inherit llvm_meta; };
+    openmp = callPackage ./openmp { inherit llvm_meta; };
 
     libunwind = callPackage ./libunwind ({ inherit llvm_meta; } //
       (lib.optionalAttrs (stdenv.hostPlatform.useLLVM or false) {