Fix clisp cross-compile

Fix clisp cross-compile

Problems:

* least-positive-long-float is unrepresentable in clisp, ending up
  with being zero
* clisp's decode-float returns a 0 exponent for a 0 float (rather
  than something large and negative)
* everything got constant-folded in the cross-compiler, thus using
  the host's runtime.

Solution: compute the value using sb!vm:double-float-digits/bias