AMDGPU: add hotswap entry trampoline core

[harsh-amd]

Stack:

1. This PR - core hotswap entry trampoline implementation, unit coverage, and minimal public LIT coverage.
2. AMDGPU: add hotswap entry trampoline lit coverage #3098 - expanded LIT coverage layered on this PR.

The temporary ROCm base branch users/harsh/comgr-hotswap-entry-core mirrors this PR head so #3098 shows only the incremental LIT diff.

Summary:

• Adds gfx12.5 hotswap entry trampoline generation and descriptor fixups.
• Adds ELF helpers for enumerating/updating kernel descriptors and SGPR metadata.
• Wires the rewrite pipeline to fail closed when requested entry trampolines cannot be installed.
• Adds focused unit coverage for the ELF helpers, trampoline builder, descriptor rewrites, SGPR metadata updates, idempotency, and failure paths.
• Adds minimal public LIT coverage for enabled entry trampolines and fail-closed scratch-SGPR exhaustion.

Verification:

• Rebased onto amd-staging at 9eeedba.
• cmake --build build --target HotswapMCTests HotswapElfTests hotswap-rewrite -- -j8 passed.
• build/bin/HotswapMCTests passed: 36 tests.
• build/bin/HotswapElfTests passed: 13 tests.
• build/bin/llvm-lit -sv build/tools/comgr/test-lit --filter hotswap-kernel-entry-trampoline passed: 1 test on this PR.
• build/bin/llvm-lit -sv build/tools/comgr/test-lit --filter hotswap passed: 53 tests on the stack top.
• git diff --check rocm/amd-staging...HEAD passed on the stack top.
• The final stack tip remains tree-identical to the previous full top-of-stack before moving minimal LIT coverage into this PR.