Add flag alias for AdaptiveCpp

[tom91136]

Currently for AdaptiveCpp we need use -DSYCL_COMPILER=HIPSYCL, we should keep supporting that but add an alias such that -DSYCL_COMPILER=ACPP works.

[illuhad]

It might be worth considering -DSYCL_COMPILER=ADAPTIVECPP. SYCL-Academy uses a similar convention (https://github.com/codeplaysoftware/syclacademy/blob/e4fec8445066e601d0e398acec866b1691e86771/CMakeLists.txt#L28) and in cmake, you also find_package(AdaptiveCpp).