Make no-float policy explicit

Summary:
We don't want floating types for various reasons, e.g.,

• Their representation and often their behavior is implementation-defined.
• Many targets don't support them.

This is a backport of libsecp256k1 PR684

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5367