linux-arm: unroll loops that may benefit

Summary: This option is *not* the same as -funroll-all-loops. Let's unroll any loop that may benefit according to gcc.

Test Plan: test stability and please report back.

Reviewers: randall.rushing, Mustaavalkosta, michyprima, Roach, synergy

Reviewed By: synergy

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

Change-Id: I684dca506753f81c05fffb330632007eb8243f88