add static context object which has no capabilities

Summary:

• prevent attempts to modify secp256k1_context_no_precomp

This is a backport of secp256k1's PR553

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4971