[SYCL][CMake] Garbage-collect in libsycl linking (#18293)
This commit adds the -Wl,--gc-sections flags to the linking of the SYCL
libraries when available. This enables link-time section garbage
collection, removing dead code in the binary, potentially reducing the
size of it.
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Signed-off-by: Larsen, Steffen <steffen.larsen@intel.com>
