[CMake] Accept legacy 'hipsycl' as deprecated alias for 'adaptivecpp'

[zjin-lcf]

Summary

Addresses #711.

hipSYCL was renamed to AdaptiveCpp, and #699 updated ONEMATH_SYCL_IMPLEMENTATION (and all internal references / docs) to use adaptivecpp. As a result, users following older instructions or scripts that pass -DONEMATH_SYCL_IMPLEMENTATION=hipsycl now hit a fatal error:

SYCL implementation hipsycl is not known

This PR accepts the legacy hipsycl value as a deprecated alias for adaptivecpp, emitting a deprecation warning that points users to the new value, so existing build scripts keep working:

if (ONEMATH_SYCL_IMPLEMENTATION STREQUAL "hipsycl")
  message(WARNING "ONEMATH_SYCL_IMPLEMENTATION=hipsycl is deprecated: ... Treating it as 'adaptivecpp'. Please use -DONEMATH_SYCL_IMPLEMENTATION=adaptivecpp instead.")
  set(ONEMATH_SYCL_IMPLEMENTATION "adaptivecpp")
endif()

Note: the original report (v0.9) predates #699; on current develop the docs and CMake already agree on adaptivecpp. This change only adds backward compatibility for the old spelling.

Test plan

• -DONEMATH_SYCL_IMPLEMENTATION=adaptivecpp unchanged.
• -DONEMATH_SYCL_IMPLEMENTATION=hipsycl (and hipSYCL, via existing TOLOWER) now maps to AdaptiveCpp with a deprecation warning instead of failing.
• Unknown values still produce the existing fatal error.

[sknepper]

Looks good - it makes sense to accept the hipsycl alias to preserve backward compatibility for existing build scripts.

[zjin-lcf]

Thanks.