i386: Add SGX CPUID leaf FEAT_SGX_12_0_EAX

CPUID leaf 12_0_EAX is an Intel-defined feature bits leaf enumerating
the CPU's SGX capabilities, e.g. supported SGX instruction sets.
Currently there are four enumerated capabilities:

    - SGX1 instruction set, i.e. "base" SGX
    - SGX2 instruction set for dynamic EPC management
    - ENCLV instruction set for VMM oversubscription of EPC
    - ENCLS-C instruction set for thread safe variants of ENCLS

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Yang Zhong <yang.zhong@intel.com>
Message-Id: <20210719112136.57018-8-yang.zhong@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>