i386: Add SGX CPUID leaf FEAT_SGX_12_1_EAX

CPUID leaf 12_1_EAX is an Intel-defined feature bits leaf enumerating
the platform's SGX capabilities that may be utilized by an enclave, e.g.
whether or not an enclave can gain access to the provision key.
Currently there are six capabilities:

   - INIT: set when the enclave has has been initialized by EINIT.  Cannot
           be set by software, i.e. forced to zero in CPUID.
   - DEBUG: permits a debugger to read/write into the enclave.
   - MODE64BIT: the enclave runs in 64-bit mode
   - PROVISIONKEY: grants has access to the provision key
   - EINITTOKENKEY: grants access to the EINIT token key, i.e. the
                    enclave can generate EINIT tokens
   - KSS: Key Separation and Sharing enabled for the enclave.

Note that the entirety of CPUID.0x12.0x1, i.e. all registers, enumerates
the allowed ATTRIBUTES (128 bits), but only bits 31:0 are directly
exposed to the user (via FEAT_12_1_EAX).  Bits 63:32 are currently all
reserved and bits 127:64 correspond to the allowed XSAVE Feature Request
Mask, which is calculated based on other CPU features, e.g. XSAVE, MPX,
AVX, etc... and is not exposed to the user.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Yang Zhong <yang.zhong@intel.com>
Message-Id: <20210719112136.57018-10-yang.zhong@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>