crypto/internal/fips140/bigmod: fix extendedGCD implementation mismatch

[ArquintL]

The existing extendedGCD implementation differs from
BoringSSL & Fiat-Crypto, which this PR fixes.

Avoiding the calls to Nat.Add in the loop results in an on average
24% speedup due to in-place updates instead of allocating temporary
Nat instances as shown in this plot:

https://github.com/user-attachments/files/27047498/bench_plot.pdf

While all annotations have been removed for this PR, the fixed
implementation is backed by proofs using Gobra (https://gobra.ethz.ch),
a program verifier for Go programs. The linked issue describes the
proofs in more details.

Fixes #78218

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[ArquintL]

cc @FiloSottile

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