[perl #70337] perldiag: localtime(...) too large

[perl #70337] perldiag: localtime(...) too large

The message ‘localtime(...) too large’ is not documented in perldiag.
The attached patch adds it. I know it’s technically ‘%s(%.0f) too
large’, but it’s easier for users to find it under ‘localtime’ and
‘gmtime’. I also had to modify diagnostics.pm to support %.0f, and I
added a test, but I used a hack with STDERR that would make debugging
very difficult to anyone else adding tests.