Amend 0529

[zz-sol]

• In impl SIMD-0529 big int mod exp syscall anza-xyz/agave#12321 we slightly changed the order of cu is charged, as per comment impl SIMD-0529 big int mod exp syscall anza-xyz/agave#12321 (comment) , as a result it does not match the spec.
• this PR consolidated the changes so the spec and implementation matches

[simd-bot]

Hello zz-sol! Welcome to the SIMD process. By opening this PR you are affirming that your SIMD has been thoroughly discussed and vetted in the SIMD discussion section. The SIMD PR section should only be used to submit a final technical specification for review. If your design / idea still needs discussion, please close this PR and create a new discussion here.

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[zz-sol]

manually tag @samkim-crypto for review