split references for ldexp out into independent files

The existing __strong_reference's in src/s_scalbn*.c are not being found since
those files get skipped in favor of the arch-specific assembly versions.
And on Windows, ldexp* needs to be defined with a DLLEXPORT to be resolved.

Fixes https://github.com/JuliaLang/julia/issues/6777

This could be made platform-dependent, using a __strong_reference on Unix and this DLLEXPORT version only on Windows, if there are performance implications of these being functions.