[MC] Pass through .code16/32/64 and .syntax unified for COFF

[MC] Pass through .code16/32/64 and .syntax unified for COFF

These flags should simply be passed through to the target, which will do
the right thing. Add an MC/X86 test that uses these directives with the
three primary object file formats and shows that they disassemble the
same everywhere.

There is a missing test for .code32 on Windows ARM, since I'm not sure
exactly how to construct one.

Fixes PR43203

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@370805 91177308-0d34-0410-b5e6-96231b3b80d8