Improve constant-timeness on PowerPC

Summary:

• Remove redundant "? 1 : 0" after comparisons in scalar code

This prevents GCC from generating branches on PowerPC in certain
cases.

Fixes #771.

• Suppress a harmless variable-time optimization by clang in _int_cmov

Follow up on 52a03512c1d800603b5c923c1a28bdba12dadb30

This is a backport of libsecp256k1 PR772

Depends on D7590

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7597