Wrap exports in eval-when.

 src/compiler/generic/objdef.lisp::
 src/compiler/generic/utils.lisp:;
 src/compiler/generic/vm-macs.lisp::
 src/compiler/sparc/c-callback.lisp::
 src/compiler/sparc/parms.lisp::

  When export lost its compile-time effects and became a normal
  function, the exports in many files no longer took affect while
  compiling the file.  This change makes the compile-time effects
  happen as before in selected files. With this change, the sparc port
  can be cross-compiled from x86 again.

 src/tools/cross-scripts/cross-x86-sparc.lisp::
  Need to frob CHAR-BYTES, which is needed by BYTE-BASH-COPY. .