[SYCL] Updated SYCL device filtering

[OuadiElfarouki]

This is a simple change to the initialization approach of the "filter" value when picking the "preferred platform" in its current form.

Given that there is no direct access to the GGML_SYCL_TARGET at this level, we can use the default_selector_v as an initial hint when the ENV variable is not specified (default_selector_v selects the level-zero by default when using the default intel target, and properly selects CUDA target otherwise given the target triplets we have setup in the Cmake). - One more motivation behind this is that the default selector is added to _devs prior to the filtering step -

This patch makes it thus possible to pick the right platform without necessary specifying the ONEAPI_DEVICE_SELECTOR for non-intel targets, and also keeps the SYCL backend from selecting vendor-heterogeneous platforms in the same setup.

Successfully tested with/without the ENV selector, on multiple platforms and targets.

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[joeatodd]

Ping @abhilash1910 for info

[abhilash1910]

Thanks @joeatodd @OuadiElfarouki for adding this!