Simple make script

I added a simple script called "makellvm" in the base llvm directory to
build a library and an executable with one command. The script runs gnumake
in the current directory, and if that completes successfully, it chdir's to
your llvm/tools/<toolname> and runs gnumake there. <toolname> can be
specified on the command name and if not, it defaults to opt.

Examples:
  makellvm ## run gnumake in $cwd, then in tools/opt
  makellvm llc ## run gnumake in $cwd, then in tools/llc
  makellvm -h ## print usage message

--Vikram