target/i386: pass X86CPU to x86_cpu_get_supported_feature_word

target/i386: pass X86CPU to x86_cpu_get_supported_feature_word

This allows modifying the bits in "-cpu max"/"-cpu host" depending on
the guest CPU vendor (which, at least by default, is the host vendor in
the case of KVM).

For example, machine check architecture differs between Intel and AMD,
and bits from AMD should be dropped when configuring the guest for
an Intel model.

Cc: Xiaoyao Li <xiaoyao.li@intel.com>
Cc: John Allen <john.allen@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>