x86: Emit LOCK prefix after DATA16

Summary: x86 allows either ordering for the LOCK and DATA16 prefixes, but using GCC+GAS leads to different code generation than using LLVM. This change matches the order that GAS emits the x86 prefixes when a semicolon isn't used in inline assembly (see tc-i386.c comment before define LOCK_PREFIX), and helps simplify tooling that operates on the instruction's byte sequence (such as NaCl's validator). This change shouldn't have any performance impact.

Test Plan: ninja check

Reviewers: craig.topper, jvoung

Subscribers: jfb, llvm-commits

Differential Revision: http://reviews.llvm.org/D6630