Emit code16 in assembly in 16-bit mode

Summary:
When generating assembly using -m16 we must explicitly mark it as
16-bit. Emit .code16 at beginning of file. Fixes wrong results when
using -fno-integrated-as.

Reviewers: dwmw2

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D19392

llvm-svn: 267152