Rather than requiring users to pass `-m64` to the `llvm-ml` driver to
get 64-bit behavior, we add the `llvm-ml64` alias, matching the behavior
of `ML.EXE` and `ML64.EXE`.
The original flavor/bitness flags still work, but the alias should make
some workflows easier.
NOTE: The logic for this already existed in the code; we're just finally
adding the build/install instructions to match.
3c657ceef9