[ARM] libc: define return and conditional return macros

Summary: Required by the new strncat, strlcat, strncmp, and strnlen

Test Plan: verified

Reviewers: randall.rushing, jriordan001, anders3408, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D239

Change-Id: Ia77f7cfd69076a9ceb65d13279c8768add4dbeec