fix(hotswap): gate entry trampoline rewrites for gfx12.5

[harsh-amd]

Stack

Layer 2 of the rocm-systems hotswap stack, paired with ROCm/llvm-project#3008.

• Previous layer: [hotswap] Install and link libhsa-hotswap tool #7577
• This layer: rocm-systems loader gating and COMGR request selection for the gfx12.5 opt-in path
• Final layer: [rocr] Remove loader-side gfx125x entry trampolines #7592
• Layer-only diff: harsh-amd/rocm-systems@hotswap-revert-7342...hotswap-entry-trampoline-gating

GitHub targets this PR at develop, so the default Files view includes prior stack layers. Use the layer-only diff above for review of this layer.

Summary

• Keep rocm-systems limited to loader routing and COMGR source/target ISA-pair construction; COMGR owns validation and all code-object transformations after the call.
• Rename the local policy unit to hotswap_loader_policy and keep gfx12.5 matching plus gfx1250 stepping-feature construction private to that unit.
• Treat AMD_COMGR_HOTSWAP_ENTRY_TRAMPOLINES as an opt-in for rocm-systems to attempt the gfx12.5 same-processor COMGR request path; COMGR owns what the flag enables internally.
• Follow Install Kernel Trampolines #7581 by keeping the opt-in path keyed to the code-object ISA rather than using this path for processor retargeting.
• Cover concrete gfx125* targets and gfx12-5-generic while keeping non-gfx12.5 agents and source code objects unchanged.
• Preserve the existing gfx1250 A0 source/target ISA pair and leave pass selection to COMGR.
• Add unit and loader-path coverage for the local routing behavior without testing COMGR pass internals.

Testing

• cmake -S projects/hotswap -B build-hotswap-reorder-layer2 -DCMAKE_BUILD_TYPE=RelWithDebInfo -Damd_comgr_DIR=/home/harsh/llvm-pr3000-refactor/build-comgr-displacement-shared/lib/cmake/amd_comgr -Dhsa-runtime64_DIR=/opt/rocm-7.2.0/lib/cmake/hsa-runtime64
• cmake --build build-hotswap-reorder-layer2 --target hsa-hotswap hotswap_test hotswap_tool_test hotswap_loader_test --parallel 32
• ctest --test-dir build-hotswap-reorder-layer2 --output-on-failure
• cmake --build build-hotswap-review --target hotswap_tool_test hotswap_loader_test --parallel 32
• ctest --test-dir build-hotswap-review -R 'hotswap_(tool|loader)_test' --output-on-failure
• git diff --cached --check

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