utf8.c: Move static function to embed.fnc

This automatically generates assertions for pointer arguments, etc.
utf8.c: Silence compiler warning

This was brought to my attention by Jarkko Hietaniemi.  The compiler was
complaining that a variable could be used uninitialized.  In practice
this doesn't happen, as it would only happen on bad data, and Perl
itself generates the data used.  (I suppose if the data got corrupted,
it could happen.)  This commit initializes the value unconditionally,
which allows a conditional setting of it to be removed.
Remove deprecated 'PL_sv_objcount'

This was scheduled to be removed in 5.20, but was forgotten.
mktables: Add regenerated header to file

This file is generated by mktables, so any changes made to it are lost.
Add the standard header for such files to it.
Revert "mktables: Remove obsolete sort constraint"

This reverts commit cf13ddc5efb78ce6d588441978947303e5baf1d8,
and adds a comment as to why.