[backport#11140] Trivial: Improve #endif comments

Summary:
ac1cf8d Trivial: Improve #endif comments (danra)

Pull request description:

Improve the #endif comments for the '#if HAVE_DECL_BSWAP_XX == 0' preprocessor conditions, so each shows the full condition which it closes.

Backport of Core PR11140

Test Plan:

ninja

Reviewers: #bitcoin_abc, PiRK, Fabien

Reviewed By: #bitcoin_abc, PiRK, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8172