Add :alien-callback to *faatures* for platforms that support it.

bootfiles/20d/boot-2012-12-1.lisp::
  Add :alien-callback to *features*

bin/build.sh::
  Change bootstrap directory to 20d.

code/lispinit.lisp::
  * Register :alien-callback feature if enabled.
  * Heap overflow checking depends on gencgc, so register that only if
    both are enabled.

tools/comcom.lisp::
  Compile c-callback only if :alien-callback is a feature.

tools/worldcom.lisp:
  Compile alien-callback onlf if :alien-callback is a feature.