`less-safe-getrandom-custom-or-rdrand` not working on target `x86_64-fortanix-unknown-sgx`

[DragonDev1906]

I'm not sure about the current state of support for this target (see #775), but I'm not sure if you know about this already, hence this issue:

The x86_64-fortanix-unknown-sgx target has target_os = "unknown" instead of none for some reason. This means that although ring compiles for this target there is no way to use ring with RDRAND. The following feature flag + line works for embedded devices but not in SGX.

all(feature = "less-safe-getrandom-custom-or-rdrand", target_os = "none"),

Thus SystemRandom can never implement SecureRandom on that target.

Also see #744

[briansmith]

Closing this as a dupe of #775. I think now we're in a good position to do the SGX port, but it requires more work than getting ring::rand to build.