libm: sync with upstream freebsd

Summary:
New long double functions have been included,
but are not in use due to the android double long
handling. imprecise.c is not built for the same reason.

Test Plan: verified

Reviewers: randall.rushing

Differential Revision: http://review.codefirex.com/D232

Change-Id: I9bbf62846404f8f40cfdfa76953b383b9c1ded33