fix Issue 12412. CTFE-able math

Issue:
https://issues.dlang.org/show_bug.cgi?id=12412

Make isNaN, isNormal, isSubnormal, isFinite, isInfinity CTFE-able.
I'd rather that you don't remove the runtime tests. These were put here because the compiler - at least gdc - was able to const-fold the tests using literals above.