This commit allows setting the AVX, AVX512, and MPX bits in SIGSTRUCT.xfrms
dynamically unless the features are specifically enabled in the manifest via
sgx.require_[avx|avx512|mpx]=1 options. If the bits are set in SIGSTRUCT.xfrms
but the CPU feature(s) are not available on the platform, the enclave
initialization (EINIT) should fail. In addition, the xfrms and xfrmmask fields
in the EINITTOKEN must match with SIGSTRUCT.