Update for ROCm 6.4.3

This is a set of changes to migrate ectrans to ROCm 6.4.3. I'm not sure why it didn't show up on the transition tracker, but it looks like you've had some issues due to the ROCm 5.7 -> ROCm 6.4 transition. That's my fault. Sorry. I'll try to ensure the ROCm 6 -> ROCm 7 transition goes more smoothly.

1. Prevent FTBFS errors caused by using clang-17 without explicitly specifying it as a B-D (which can occur when the backing compiler for hipcc changes). As of hipcc 7.0.2+dfsg-2, the default HIP compiler is autodetected by cmake as does not need to be specified. See also https://bugs.debian.org/1118435.

2. Use rocm-target-arch to choose the AMD GPU target list. There is typically also a custom field set on the library. I'm unsure of which binary package put it as it's not obvious to me where the GPU code compiled during the build is going. In any case, this is the field that is usually put on library packages containing AMD GPU machine code:

XB-X-ROCm-GPU-Architecture: ${rocm:GPU-Architecture}

3. Drop HIP backend for ppc64el. There have been compiler bugs blocking the updates to hipfft and hipblas on ppc64el. I'm strongly considering dropping ppc64el entirely as a build target for the ROCm math libraries on Debian.