Undef HAVE___INT128 in basic-config.h to fix gen_context compilation

Summary:
Fixes #768.

This is a backport of libsecp256k1 PR769

Test Plan:

ninja check-secp256k1

That doesn't really cover the change, but I really do nto have the setup to repro. At least it doesn't break anything.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7593