Avoid std::locale/imbue in DateTimeStrFormat

Summary:

And replace them with just hardcoded ISO8601 strings and gmtime_r.

Pointed out by @laanwj here: #12970 (comment)

Backport of core PR12973 and PR13031
https://github.com/bitcoin/bitcoin/pull/12973/files
https://github.com/bitcoin/bitcoin/pull/13031/files

The build for MSVC is left untested and just backported "as-is". AFAIK
we don't support MSVC build and have no plan to do so, but in the event
this should occur in the future that part of code will be ready.

Test Plan:

make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3649