amrwbenc: Fix minor UAL/LLVM AS issues

Summary: .end directive is now supported in LLVM as a GAS compatibility, however only .end is supported, not .END (which GAS handles the same).

Test Plan: verified

Reviewers: randall.rushing, jriordan001, anders3408, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D450