Add experimental support for SYCL under an experimental gate. The
IntelLLVM and Adaptivecpp compilers are recognized, as well as upstream
Clang with libsycl support.
Fixes: #27219, #27905
Provide it whenever we can detect a target architecture from the
compiler during compiler inspection. In order to avoid changing
existing IDs, do this only for compilers where we don't already
detect a target architecture during compiler identification.
Fixes: #17702
Vito Gamberini