Support flushing the L2 cache before each invocation

[Eyal Rozenberg]

At the moment, the kernel invocations begins with an caches which may well contain much of the data used in the previous invocation. While this also happens in real life sometimes - it is often better to time kernel execution after flushing the caches.

We don't have the ability of fully flushing all caches (except perhaps with a device reset); but it seems we can flush at least the L2 cache - by allocating a large enough buffer and memset'ing it to 0. I hope it works.