fix: preserve AUIPC AOT immediates

[Kuhai9801]

Fixes #2851.

Summary

RV32IM AUIPC AOT now uses the raw 24-bit transpiled immediate when computing pc + (imm << 8), instead of narrowing the immediate through i16.

Testing

• OPENVM_FAST_TEST=1 OPENVM_SKIP_DEBUG=1 cargo test -p openvm-rv32im-circuit --features aot aot_uses_full_24_bit_auipc_immediate
• cargo +nightly fmt --all -- --check
• cargo clippy -p openvm-rv32im-circuit --features aot --all-targets -- -D warnings

[GunaDD]

I agree with you that the issue is valid and the fix is correct. Thank you for this! Could you undo the change in the isa.mdx file? Other than that, LGTM.