[SECP256K1] Increase precision of g1 and g2
[SECP256K1] Increase precision of g1 and g2

Summary:
```
This allows us to shift by 256+128 = 384 bits, which is a multiple of
the limb size of the scalar representation. This also happens to be the
most precision possible for g2 that still fits into a 256-bit value.
```

Partial backport 2/11 of secp256k1 [[https://github.com/bitcoin-core/secp256k1/pull/830 | PR830]]:
https://github.com/bitcoin-core/secp256k1/pull/830/commits/76ed922a5f09d63e0622825ca83d9301c1ef3efe

Test Plan:
  ninja check-secp256k1

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8038

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