Use C limits for foreign floats and doubles instead of Lisp's.

- Export 'float_min', 'float_max', 'double_min', and 'double_max' from
  the libtest shared library with the values of FLT_MIN, FLT_MAX,
  DBL_MIN, and DBL_MAX, respectively.
- Use *FLOAT-MIN*, *FLOAT-MAX*, *DOUBLE-MIN*, and *DOUBLE-MAX* as test
  values for foreign floats/doubles instead of the Lisp constants
  <foo>-POSITIVE-<bar>-FLOAT.