x86: realmode: Workaround clang issues

clang doesn't properly support .code16gcc and generates wrong machine
code [1][2][3][4]. But the test works if object file is compiled with -m16 and
explicit suffixes are added for instructions.

1. https://lore.kernel.org/kvm/4d20fbce-d247-abf4-3ceb-da2c0d48fc50@redhat.com/
2. https://lore.kernel.org/kvm/20200915155959.GF52559@SPB-NB-133.local/
3. https://lore.kernel.org/kvm/788b7191-6987-9399-f352-2e661255157e@redhat.com/
4. https://lore.kernel.org/kvm/20200922212507.GA11460@SPB-NB-133.local/



Suggested-by: Thomas Huth <thuth@redhat.com>
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>
Message-Id: <20200924120516.77299-1-r.bolshakov@yadro.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>