msun: Silence new harmless -Wimplicit-int-float-conversion warnings

[jrtc27]

Summary

Clang from trunk recently added a warning for when implicit int-to-float
conversions cause a loss of precision. The code in question is designed
to be able to handle that, so add explicit casts to silence this.

[dim]

LGTM. I guess we could also define DYPE_MIN and DTYPE_MAX themselves to have the casts, but that looks like more churn.