[secp256k1] refactor: Rename file that contains static ecmult_gen table

Summary:
This is a partial backport of secp256k1#988
https://github.com/bitcoin-core/secp256k1/pull/988/commits/8ae18f1ab3dce4c487bab75c2f0cdf4fe311b318
https://github.com/bitcoin-core/secp256k1/pull/988/commits/9ad09f6911906a1fa9af2c5540a8004e44f3ccc6
Depends on D19385

Test Plan:

cmake .. -GNinja -DSECP256K1_ECMULT_STATIC_PRECOMPUTATION=1
ninja secp256k1
ls src/secp256k1/src/

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D19386