Add chipStar/SPIRV HIP backend and SYCL 2020 include fixes

[pvelesko]

Summary

• cmake: Teach FindHIP.cmake to recognize HIP_PLATFORM=spirv (chipStar), finding libCHIP.so and defining __HIP_PLATFORM_SPIRV__ for host compilation
• hip: Replace runtime arch-string heuristics in getDeviceArch and setArchCompilerFlags with compile-time __HIP_PLATFORM_AMD__ / __HIP_PLATFORM_NVIDIA__ / __HIP_PLATFORM_SPIRV__ branches
• dpcpp: Update SYCL include path from legacy <CL/sycl.hpp> to SYCL 2020 <sycl/sycl.hpp> (required by oneAPI 2025+)
• tests: Remove using int2/int3 aliases that collide with chipStar's HIP vector type definitions

Background

chipStar is an open-source HIP implementation that compiles kernels to architecture-agnostic SPIR-V IR, targeting Intel GPUs via Level-Zero/OpenCL. SPIR-V differs from AMD/NVIDIA HIP in two key ways relevant to OCCA:

• No --offload-arch flag is needed (SPIR-V is ISA-agnostic; the GPU driver JIT-compiles at load time)
• gcnArchName in hipDeviceProp_t returns "spirv64" rather than a GPU microarchitecture string

The chipStar-side fixes (implementing hipModuleLoad from file, populating gcnArchName, and suppressing unknown --offload-arch flags in hipcc) are tracked separately in the chipStar repo.

Test plan

• All 13 HIP (chipStar) examples pass: ctest -j1 -R hip
• Builds cleanly with HIP_PLATFORM=spirv against chipStar install
• Verify AMD HIP build is unaffected (no SPIRV-specific ifdefs in non-CMake code paths)