Add function to print a description of the value of the FPSCR
(floating-point status and control register).
Partially addresses ticket:91 by allowing the test suite to finish.
The tests still fail (along with others), but the test suite will
finish.

 * code/float-trap.lisp:
   * Update SET-FLOATING-POINT-MODES:
     * When we clear out bits in float-invalid-op-1-byte, clear out the
       float-invalid-op-2-byte.  Not necessary for this fix, but those
       bits signal other invalid operations from sqrt and such.
     * When clearing the exceptions, clear the sticky exceptions
       summary bit too.
   * Fix SIGFPE-HANDLER:
     * As above, clear out the bit in float-invalid-op-2-byte and the
       exceptions summary bit.
     * Only clear out the new exception bits, as done for SSE2.
     * Mask out the float-invalid-op-2-byte and the summary bit before
       setting the new modes.

 * compiler/ppc/parms.lisp
   * Define float-exceptions-summary-byte.

 * code/exports.lisp:
   * Export FLOAT-INVALID-OP-2-BYTE