[v3] Remove constexpr from frexp, modf, remquo

[Paolo Carlini]

Hi,

Daniel noticed that in <cmath> we have functions marked constexpr which 
aren't really viable anyway due to the pointer parameter. Seems 
basically harmless given our way to deal with these matters, but I agree 
can be misleading. Tested x86_64-linux, committed.

Paolo.

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