Macro to include Asylo’s SGX backend dependencies in a WORKSPACE.

SGX backend dependencies have Intel’s highest level of LVI mitigation (“All-Loads-Mitigation”) applied automatically 1. This can be customized on a per-target basis by specifying one of “lvi_all_loads_mitigation”, “lvi_control_flow_mitigation”, or “lvi_no_auto_mitigation” in the list of transitive_features on the corresponding unsigned enclave target, or with a top-level --features=lvi_*_mitigation flag passed to Bazel.