[x86] Add basic support for .code16

[x86] Add basic support for .code16

This is not really expected to work right yet. Mostly because we will
still emit the OpSize (0x66) prefix in all the wrong places, along with
a number of other corner cases. Those will all be fixed in the subsequent
commits.

Patch from David Woodhouse.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198584 91177308-0d34-0410-b5e6-96231b3b80d8