Properly handle a denormal operand exception. This should get rid of

the occasional problem about SIGFPE's with no exceptions enabled.

error.lisp:
o Add floating-point-denormal-operand arithmetic error.

exports.lisp:
o Export the new arithmetic error type.

float-trap.lisp:
o Check the denormal trap bit and signal the appropriate error.

release-20c.txt:
o Update.