Rename alloca... to something else in the SYCL runtime

In Unix `alloca()` means something very precise for 40+ years and it is also used inside LLVM IR as a specific instruction https://llvm.org/docs/LangRef.html#alloca-instruction
In this SYCL implementation it is used all over the place in the runtime as a shortcut for allocation.
While `alloca()` is about allocation too, it is super misleading.
Every time time I look at a PR here with "alloca" I get bitten.
I mentioned this several times already in the past in some reviews but now I think it should be updated before it spreads too much or is vetoed during up-streaming.
"alloc" could be a good trade-off, if "allocation" is too long.
Just no "alloca" when it is not about `alloca()`. :-)