HIP: fix GPU_TARGETS

[JohannesGaessler]

When setting AMD GPU architectures via AMDGPU_TARGETS, you get the following warning:

CMake Warning (dev) at /opt/rocm/lib/cmake/hip/hip-config-amd.cmake:70 (message):
  AMDGPU_TARGETS is deprecated.  Please use GPU_TARGETS instead.
Call Stack (most recent call first):
  /opt/rocm/lib/cmake/hip/hip-config.cmake:149 (include)
  ggml/src/ggml-hip/CMakeLists.txt:41 (find_package)
This warning is for project developers.  Use -Wno-dev to suppress it.

This warning does not originate from within ggml, if you then try to set GPU_TARGETS as suggested it doesn't work because we are currently setting CMAKE_HIP_ARCHITECTURES based on AMDGPU_TARGETS only. This PR simply adds the same logic for GPU_TARGETS.

[IMbackK]

Jup that works, looks good.