libm: add fmal long double stub

Summary:
This is needed in some target cases, and has since been fixed internally in the AOSP.

From enh@google.com regarding this change:
you were right, and this was the way we should have done it, but i didn't work that out until i fixed this in the same way as part of the aarch64 work (where the weak reference didn't work).

Test Plan: verified

Reviewers: randall.rushing, jriordan001, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D234

Change-Id: I0e49f2a33cf3b178b0dae7b703b252aad7183563