musa : define __CUDA_ARCH__ for device passes (#29508)

The MUSA vendor header never defined __CUDA_ARCH__, so every architecture
test in the shared ggml-cuda sources evaluated to 0.  Kernel bodies gated on
the architecture therefore compiled to nothing, for example the q8_0 -> f16
dequantization kernel in convert.cu, whose NO_DEVICE_CODE fallback expands to
an empty body in host code.

Report the newest architecture like the HIP backend does and exclude the
NVIDIA-only features explicitly, as they are not usable on MUSA.  Define it
for device passes only: CUB uses defined(__CUDA_ARCH__) to detect device
compilation, which is also how nvcc behaves.

Drop the now-redundant defined(__CUDA_ARCH__) checks in the architecture
comparisons: __CUDA_ARCH__ is undefined in host passes for CUDA and MUSA, and
HIP defines it for every pass, so both forms select the same branch.