[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

llvm-svn: 370805