[SYCL][COMPAT] Adds sycl::length wrapper to SYCLcompat (#12968)
Adds a templated wrapper to sycl::length in syclcompat, that wraps over
`sycl::vec<ValueT,N>` up to N == 4, and calculates the length with N >
4.
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Signed-off-by: Alberto Cabrera <alberto.cabrera@codeplay.com>
