[SYCL] adding binary caching support to kernel_compiler extension

[cperkinsintel]

Trying to reuse as much of the PersistentDeviceCodeCache as possible. We use the same top cache directory as the regular binary caches, but with a ext_kernel_compiler subdirectory and a slightly different system for assigning paths.

Rather than write new tests, am just adding a "test the cache" passes to the existing tests.