Studio: forward the resolved AMD gfx arch to the prebuilt installer (#5923)

setup.sh and setup.ps1 resolve the AMD gfx target (rocminfo/hipinfo/amd-smi,
name inference, or UNSLOTH_ROCM_GFX_ARCH) but never passed it to
install_llama_prebuilt.py. The installer re-probed on its own and, on hosts where
hipinfo/amd-smi cannot report the arch (amd-smi-only Linux, HIP-runtime-only
Strix Halo, name-inferred GPUs), left rocm_gfx_target unset. The lemonade HIP
prebuilt selection needs that arch, so those hosts got no GPU prebuilt and fell
back to a source build.

Add a --rocm-gfx argument (defaulting to UNSLOTH_ROCM_GFX_ARCH) and fold it into
the host profile via _apply_host_overrides: a forwarded gfx is authoritative
(setup already applied visible-device selection) and implies has_rocm. setup.sh
and setup.ps1 now forward their resolved arch.

Add unit tests for _normalize_forwarded_gfx and _apply_host_overrides, plus
source checks that both setup scripts forward --rocm-gfx.