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

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.

Submitted by: James Clarke <jrtc27@jrtc27.com>
Reviewed by: dim
Obtained from: CheriBSD
MFC after: 1 week
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D21913