Changes to allow multiple architecture binaries to co-exist in the

Changes to allow multiple architecture binaries to co-exist in the
same file system.  In MacOS X, for example, both the i386 and x86-64
architectures can be used, so it's useful to have both available for
development work, if nothing else.

* Change primary executable name and library name to
  "mit-scheme-ARCH".

* Create a new shell script in the microcode that implements the
  decision logic for choosing a target architecture, based on the host
  architecture and the --enable-native-code argument.  Use this script
  in both the top-level configure file and the microcode configure
  file, so that they agree on the architecture and the corresponding
  names.

* Add logic to create an executable symlink "mit-scheme-native"
  pointing to a native-code executable.  This name is used when we
  need a native-code implementation but don't care which one.  (It's
  also for compatibility.)

* Rework several shell scripts to make them more portable, using the
  autoconf shell portability recommendations.