-m16 option is available only since GCC 4.9.0 [1]. That causes a build
failure on centos-7 [2] that has GCC 4.8.5.

Fallback to -m32 if -m16 is not available.

1. http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59672
2. https://gitlab.com/bonzini/kvm-unit-tests/-/jobs/755368387

Fixes: 2616ad93

 ("x86: realmode: Workaround clang issues")
Reported-by: Roman Bolshakov <r.bolshakov@yadro.com>
[Commit message by Roman Bolshakov, patch redone. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>