{f32,f64,float}::tests::test_frexp unit tests in libstd fail on Windows

[vadimcn]

SImilarly to #8663, looks like mingw32 implementation of frexp() violates the spec when dealing with +/-infinity.

[alexcrichton]

This has not been fixed by upgrading to mingw-w64

[steveklabnik]

@retep998 do you know if this is still an issue? I'm not aware of tests failing.

[retep998]

I have no idea. I'm not an expert on the precise behavior of the math functions.

[steveklabnik]

I guess I meant if you had seen test failures, not the details of them failing.

[steveklabnik]

Given that there's been no substantial comments here in years, I'm going to consider this one stale; please let me know if this is somehow reproducible today.