Give private macros a more unique name

There are various macros in the public headers that might have conflicts with user code or other libraries.
For example: `ALWAYS_INLINE`.
There should have a more defensive name, such as `__SYCL_ALWAYS_INLINE`.
Actually, since we might have different SYCL implementations used at the same time, perhaps something like `__DPCPP_ALWAYS_INLINE` is better.