i386: Add SGX CPUID leaf FEAT_SGX_12_0_EBX

CPUID leaf 12_0_EBX is an Intel-defined feature bits leaf enumerating
the platform's SGX extended capabilities.  Currently there is a single
capabilitiy:

   - EXINFO: record information about #PFs and #GPs in the enclave's SSA

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Yang Zhong <yang.zhong@intel.com>
Message-Id: <20210719112136.57018-9-yang.zhong@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>