Implementation of SYCL_KHR_QUEUE_FLUSH

[TApplencourt]

Hi,

I implemented SYCL_KHR_QUEUE_FLUSH.

As SimSYCL use greedy execution by default, this is a no-op. (Please note that I didn't add any tests because of that)

Thanks!

And please excuse me in advance for any problem in this PR : )

Regards,
Thomas

[PeterTh]

Hey Thomas!
As it is a no-op not just due to implementation specifics but really by design in SimSYCL, I think not having specific tests is fine.

However, we've been discussing this internally, and also due to the nature of SimSYCL, it would be good to have the ability to enable/disable support for extensions. E.g. imagine a use case of trying to create CI test coverage for platforms that support a given extension and platforms which don't support it, to validate both paths in your software.

I think it would make a lot of sense to derive the option name for enabling/disabling from the feature test macro name of the extension. For example, for this case it could be SIMSYCL_ENABLE_KHR_QUEUE_FLUSH.

I've implemented this mechanism here.
While the extension is extremely simple for SimSYCL and doesn't need testing of its semantics, I didn't trust the somewhat more complicated option mechanism so I also implemented dedicated CI for that.

I first wanted to push this as an update for this PR, but I couldn't do that, so I opened that new PR for it.