date: align Thai locale formatting with GNU

[compilersutra]

Fix Thai locale formatting so GNU-compatible output is preserved:

• keep %Y Gregorian under th_TH.UTF-8
• format %EY, %EC, and %Ey with Buddhist-era values
• use locale-provided month/day names instead of hardcoded ICU fallbacks
• skip locale-name tests when the locale is not installed locally

Validation:

• cargo test --test tests test_date_thai_locale_solar_calendar -- --nocapture
• cargo test --test tests test_locale_calendar_conversions -- --nocapture
• cargo test --test tests

Full suite on this machine: 4666 passed, 0 failed, 40 ignored

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/date/date-thailand. tests/date/date-thailand is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/rm/isatty (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/retry (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/printf/printf-surprise is now passing!

[compilersutra]

I found a conflict with tests/date/date-thailand.sh: GNU expects Thai solar-calendar output for date +%Y and Thai month names, but this patch makes the Thai locale Gregorian/
English for -d too. Should I narrow the fix to preserve the GNU test behavior, or is changing that GNU test expected?

[sylvestre]

You should match what gnu is doing. Thanks

[github-actions]

Binary size comparison:

Individual binary size comparison VS main (threshold: >=5% AND >=4 KB).

Total size of compared binaries: 150.39 MB (-1.03 MB, -0.68%)

Significant per-binary changes:
  date     2.72 MB ->    1.73 MB  (-1008 KB, -36.21%)