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.