Enable LVI mitigation for SGX

**Is your feature request related to a problem? Please describe.**

To harden the enclaves, it would be good to protect them against LVI attacks.

**Describe the solution you'd like**

Follow the steps in Section "Enable CVE-2020-0551 Mitigation" of the "Enclave Development Basics" Chapter (page 86ff in the [Intel® Software Guard Extensions (Intel® SGX) SDK for Linux* OS -- Developer Reference ](https://download.01.org/intel-sgx/latest/linux-latest/docs/Intel_SGX_Developer_Reference_Linux_2.10_Open_Source.pdf))
[Note: this relies for ubuntu 18.04 on the upstream binutils For docker, we install intel's version distributed as part of the SDK in `/opt/intel/sgxsdk.extras/external/toolset/ubuntu18.04` ...]