Create SGX build

[axic]

The SDK can be found here: https://software.intel.com/content/www/us/en/develop/topics/software-guard-extensions/sdk.html

The SDK contains installers for various distros, including recent Ubuntu versions, so should we do this task it would make sense installing it on one of the ethereum/cpp-build-env images.

A basic guide is here, and the official one here.

I think the work done in #293 would be a good stepping stone as it requires very similar changes/improvements (no pthreads/dlopen/etc).

[kss-espeo]

A note from me: I currently need exactly that, and I suppose there is more and more SGX development happening these days so it's worth doing it :)

For my build I decided to package evmone into https://github.com/openenclave/openenclave . You might want to consider that, too.

Anyway, if you have any pointers on how to go about this, it would be highly appreciated.