[ARM NEON] libm: original optimized modf

Summary: Significantly faster than the generic C implementation

Test Plan: test on a device that had issues with libm neon based modf implementation.

Reviewers: anders3408, jriordan001, raymanfx, randall.rushing

Reviewed By: randall.rushing

CC: raymanfx

Differential Revision: http://review.codefirex.com/D496