[SYCL] remove global variables (#7710)

[Nuullll]

• separate DPCT helpers outside

• replace global variables with context

• remove useless extra

• update mul_mat condition

• remove duplicate buft initialization

• remove duplicate extra and global work group size

• remove useless backend check

• remove duplicated extras

• use macro for group_size and remove cuda-related

[Nuullll]

The sync script didn't catch ggml-sycl folder. This is a manual sync of https://github.com/ggerganov/llama.cpp/pull/7710

[ggerganov]

Oh thanks for noticing - let me see if I can fix the sync scripts