date: fix %#P case-swap on lowercase am/pm #11659

[vishakha1411]

Fixes #11659

%P already returns lowercase am/pm. The current # handling uppercases any non-uppercase value, so %#P becomes PM. GNU keeps %#P as pm. Adjusted the case-swap logic to leave already-lowercased alt-specifier unchanged.

The test_date_strftime_case_flag_on_alt_ampm() test case now passes.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/symlink (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)

[cakebaker]

Thanks for your PR!