[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: Id0f59bdf3efafdb06f8ea588bcc08b8fdc5483c5
Signed-off-by: DHOMD <DHO@dho.im>