Rearrange everything.

Rearrange everything.

  * Have `setup.sh' define `CROSS_ARCH', which is the GNU triplet for
    the target architecture.  This is then exported and can be used by
    `config.site' to define further things.

  * Factor out the common machinery between Android and Linux
    `config.site' files.  This is, in fact, exactly the Linux
    `config.site', so have `mdw-cross' select this as a default if
    nothing more specific can be found.

  * Factor out the common machinery for Android `config.site' files,
    which was basically all of it.

  * Move the compiler flags into the new `android-cross-config.site'
    file.

  * Replace `$ANDROID_CC' by `$CROSS_ARCH-gcc'.  Factor out the
    mandatory compiler flags into `$CROSS_gcc_FLAGS', and generalize this
    so that we also set `$CROSS_gxx_FLAGS' for C++.

  * Introduce a new program `crosstool' intended to be invoked via a
    symlink, which just execs the appropriate cross-toolchain tool with
    any necessary extra flags.  Include a bunch of useful symlinks and
    have `mdw-cross' hook them onto the path.

  * Add a Makefile to build the `...-setup.sh' files for each target
    architecture from skeletons.  The Makefile is unnecessarily hairy
    but I like them that way.