Silence -Wunused-parameter my_perl under threads.

For S_ functions, remove the context.
For Perl_ functions, add PERL_UNUSED_CONTEXT.

Tricky because sometimes depends on DEBUGGING, and sometimes
on whether we are have PERL_IMPLICIT_SYS.

(Why all the mathoms Perl_is_uni_... and Perl_is_utf8_...
functions are not being whined about is a mystery.)

vutil.c (included via util.c) has one of these, but it's cpan/,
and a known problem: https://rt.cpan.org/Ticket/Display.html?id=96100
The g++ flags removal was not right.
HP-UX cc in PA-RISC2.0 optimizer crashes with some files.
Lower the optimization on Digest-SHA for HP-UX cc.

Upstream, but we are adding a platform specific hints file.
Disable ODBM (via i_dbm) in HP-UX, seems to be buggy.
Note the open RT ticket.
In case someone manages to call g++ with -Wc++-compat.
Remove old file from @writables; was taken out in autodie 2.24
Remove x2p remnants
fix a typo in perl520delta
Avoid blank lines in backslash warnings test.

This is a follow-up to 12f55bbce575aecc, which fell victim to a
bug workaround. Because the broken pipes on VMS sometimes put
multiple blank lines in test output, we collapse multiple newlines
into one.  Which breaks tests that genuinely have multiple blank
lines in the output.

So don't leave the output blank, which coincidentally makes it a
lot easier to see which print statement produces which line of
output.
Couple more (optionally) unused contexts.
GCC_DIAG_IGNORE/RESTORE whine in non-gcc if at file level.
Begin to finalize perldelta for release
Update perldelta acknowledgements
Update Module::CoreList for 5.21.1
Finalize perldelta
Add new release to perlhist
Add new perldelta for 5.21.2
Add Epipgraph for 5.21.1