[SYCL][EXT][COMPLEX] Enable std::enable_if via Return type instead of type template parameter (#10594)

Following a review of the marray complex specialization PR
(#8647), where the `std::enable_if`
are enabled via template type parameter. It has been asked to be
switched to return type instead; this PR follows the same approach to
have a coherent codebase.

Furthermore, all `std::enable_if<...>::type` has been switched to
`std::enable_if_t` for coherence.

And finally, this PR also fixes a bug where the `std::enable_if` depends
on the trait `is_gencomplex`, but should depend on `is_genfloat` since
the functions are specialized for `complex<T>`, `T` is the one that
should be tested.

Author: jle-quel