Add support for Code16GCC

[X86] The .code16gcc directive parses X86 assembly input in 32-bit mode and
outputs in 16-bit mode. Teach parser to switch modes appropriately.

Reviewers: dwmw2, craig.topper

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D20109

llvm-svn: 282430