tests/lib/libm/t_ilogb: Expand and tidy.

1. Instead of calling out VAX by name, use #ifdef NAN and
  isinf(INFINITY).  (VAX defines INFINITY even though it's not an
  infinity, not sure if there's a better compile-time test.)

2. Verify ilogbl works on long double on all architectures, not just
  those with __HAVE_LONG_DOUBLE which means long double is _larger_
  than double.

(riastradh)