number not representable as a `float` in Phobos unittest

[Paolo Invernizzi]

Hi everybody,

I'm having that when compiling a project with unittests enabled:

```
/Users/pinver/dlang/dmd-2.103.0/osx/bin/../../src/phobos/std/math/exponential.d(3791,36): Error: number `0x0.8p-126f` is not representable as a `float`
/Users/pinver/dlang/dmd-2.103.0/osx/bin/../../src/phobos/std/math/exponential.d(3791,36):        https://dlang.org/spec/lex.html#floatliteral
/Users/pinver/dlang/dmd-2.103.0/osx/bin/../../src/phobos/std/math/exponential.d(3791,75): Error: number `0x0.8p-126f` is not representable as a `float`
/Users/pinver/dlang/dmd-2.103.0/osx/bin/../../src/phobos/std/math/exponential.d(3791,75):        https://dlang.org/spec/lex.html#floatliteral
/Users/pinver/dlang/dmd-2.103.0/osx/bin/../../src/phobos/std/math/exponential.d(3793,36): Error: number `0x0.555556p-126f` is not representable as a `float`
/Users/pinver/dlang/dmd-2.103.0/osx/bin/../../src/phobos/std/math/exponential.d(3793,36):        https://dlang.org/spec/lex.html#floatliteral
/Users/pinver/dlang/dmd-2.103.0/osx/bin/../../src/phobos/std/math/exponential.d(3793,80): Error: number `0x0.555556p-126f` is not representable as a `float`
```

I've searched a little, but I don't know if it's a known bug or 
regression.

/Paolo