SYCL: Fix GGML_SYCL_DEBUG macro

[qnixsynapse]

Originally, this work was part of #11515. Currently this macro doesn't work on files other than ggml-sycl.cpp because of a variable g_ggml_sycl_debug which gets initialized as a local variable only on ggml-sycl.cpp.
This change enables the macro on other files.

I have also added debug logs to softmax OP function which I removed it then because of the mentioned issue.

[ggerganov]

Btw, what is the purpose of having GGML_SYCL_DEBUG - you can simply use GGML_LOG_DEBUG.

[qnixsynapse]

Btw, what is the purpose of having GGML_SYCL_DEBUG - you can simply use GGML_LOG_DEBUG.

It enables the backend specific logging system only when GGML_SYCL_DEBUG=1 is set in env.

I previously tried to use GGML_LOG_DEBUG but the maintainers of the SYCL backend told me that it is way too verbose to separate the backend logs from the rest of the logging system for debugging purposes.

[Alcpz]

To be fair to ggerganov's question we could rework it to have GGML_SYCL_DEBUG prepend a prefix like [SYCL] to debug messages and print stuff through GGML_LOG_DEBUG. It is fine either way for me.