Add --allow-speculation option to disable IBPB/STIBP mitigation

[KentaTada]

In https://github.com/opencontainers/runc/issues/2430#issuecomment-633763854, I considered disabling IBPB/STIBP to improve the performance impact on bytecode interpreters. This commit adds the flag which disables IBPB/STIBP mitigation for container.

Signed-off-by: Kenta Tada [email protected]

[giuseppe]

is it something worth adding to the runtime specs?

[KentaTada]

@giuseppe Thank you. I reconsidered this feature will be needed from other container runtimes and some use cases. So, I want to discuss this feature in https://github.com/opencontainers/runtime-spec/pull/1047 at first. I'll continue this PR after that.