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.