[cmake] Combine -fsycl-targets across all enabled GPU backends (fixes #708)

[zjin-lcf]

Summary

Fixes #708.

example_lapack_getrs_usm (and other USM examples) throw a SYCL No kernel named ... was found exception on an NVIDIA GPU when oneMath is built with multiple GPU vendor backends enabled at once (e.g. CUDA + Intel + AMD).

Root cause

The SYCL offload targets were set on the shared ONEMATH::SYCL::SYCL interface target in two places:

• cmake/FindCompiler.cmake used a mutually-exclusive if/elseif, so only one vendor's -fsycl-targets triple was ever selected.
• Each vendor backend's CMakeLists.txt (cublas, rocblas, cusparse) additionally appended its own -fsycl-targets flag.

DPC++ only honors the last -fsycl-targets flag on the command line, so in a multi-vendor build all but the last triple were silently dropped. The device images for the other vendor(s) were never generated, so their SYCL kernels (e.g. the getrf 32→64-bit pivot-casting parallel_for in the cuSOLVER backend) were missing at runtime.

Fix

• Assemble the offload target triples for all enabled GPU backends once, in cmake/FindCompiler.cmake, and pass them together as a single comma-separated -fsycl-targets=... list (with per-target -Xsycl-target-backend=<triple> arch options).
• Preserve the Intel GPU spir64 image when ENABLE_MKLGPU_BACKEND is combined with a CUDA/HIP backend.
• Remove the now-redundant per-backend -fsycl-targets appends from the cublas, rocblas, and cusparse CMakeLists.txt.

For a tri-vendor build this now produces e.g. -fsycl-targets=nvptx64-nvidia-cuda,amdgcn-amd-amdhsa,spir64, so every enabled vendor gets a device image. Single-vendor builds are unchanged.

Test plan

• Build with -DENABLE_MKLGPU_BACKEND + all CU* + all ROC* backends (-DHIP_TARGETS=gfx1100) as in example_lapack_getrs_usm throws SYCL exception on NVIDIA GPU when built with support for AMD GPU backends #708.
• Run example_lapack_getrs_usm on the NVIDIA GPU and confirm it no longer throws No kernel named ... was found.
• Confirm single-vendor (CUDA-only, HIP-only) builds still configure and run correctly.