Perl_newSVsv_flags_NN_PVxx: do not copy the SVprv_WEAKREF flag

[richardleach]

When copying source SV flags to the new destination SV, this function
failed to account for SVprv_WEAKREF and SVf_IVisUV flags having the
same numerical value - 0x80000000. The SVprv_WEAKREF flag was
consequently erroneously propagated when copying weakened references.

This didn't trip existing tests because SVt_IVs (the predominant SV type
for RVs) are copied using different code paths.

This commit:

• Always drops the SVprv_WEAKREF flag in the affected code path
• Adds additional tests for copying weakened SVs

---

• This set of changes does not require a perldelta entry.

[jkeenan]

I build a perl locally from the branch in this pull request, then attempted to install some of the modules named in other issues:

$ ./bin/cpanm Test::Deep DBI Sub::Quote Array::Compare Moose Dist::Zilla 

Result:

$ ./bin/perl -MTest::Deep  -MDBI  -MSub::Quote  -MArray::Compare -MMoose -MDist::Zilla -E 'say qq|hello world|;'
Prototype mismatch: sub main::blessed: none vs ($) at /home/jkeenan/testing/gh-23657-richardleach-newSVsv_flags_NN_weakref-20250901/lib/perl5/site_perl/5.43.3/Sub/Exporter.pm line 897.
hello world

I'll investigate that warning, but I don't think it's an impediment to merging this p.r. into blead. Doing so now.