make m?$pat? match only once under ithreads

[perl #115080]

m?...? is only supposed to match once, until reset. Normally this is done
by setting the PMf_USED flag on the PMOP. Under ithreads we can't modify
ops, so instead we indicate by setting the regex's SV to readonly. (This
is a bit of a hack: the flag should be associated with the PMOP, not the
regex).

This breaks with run-time regexes when the pattern gets recompiled; for
example:

    for my $c (qw(a b c)) {
	print "matched $c\n" if $c =~ m?^$c$?;
    }

outputs

    matched a

on unthreaded, but

    matched a
    matched b
    matched c

on threaded.

The re_eval jumbo fix made this more noticeable by sometimes recompiling
even when the pattern text hasn't changed (to make closures work ok).

The quick fix is to propagate the readonlyness of the old re to the new
re. (The proper fix would be to store the flag state in a pad slot
associated with the PMOP).
Needless to say, I've gone for the quick fix.