Move regex global variables to interpreter level

Move regex global variables to interpreter level

This is part of fixing gh #17154

This scenario from the ticket
(https://github.com/Perl/perl5/issues/17154#issuecomment-558877358)
shows why this fix is necessary:

    main interpreter initializes PL_AboveLatin1 to an SV it owns

    loads threads::lite and creates a new thread/interpreter which
    initializes PL_AboveLatin1 to a SV owned by the new interpreter

    threads::lite child interpreter finishes, freeing all of its SVs,
    PL_AboveLatin1 is now invalid

    main interpreter uses a regexp that relies on PL_AboveLatin1, dies
    horribly.

By making these interpreter level variables, this is avoided.  There is
extra copying, but it is just the SV headers, as the real data is kept
as static C arrays.