fix Issue 12412. CTFE-able math

Issue:
https://issues.dlang.org/show_bug.cgi?id=12412

Make isNaN, isNormal, isSubnormal, isFinite, isInfinity CTFE-able.
```D
static assert(2.0L^^(real.max_exp-1) - 1 == 2.0L^^(real.max_exp-1));//OK
```