don't segfault given string repeat count larger than 2^31

don't segfault given string repeat count larger than 2^31

E.g., this overflows INT_MAX and overruns heap memory:

    $ perl -le 'print "v"x(2**31+1)'
    [Exit 139 (SEGV)]

(Perl_repeatcpy): Use the same type for "count" as our sole
callers in pp.c: IV (long), not I32 (int).  Otherwise, passing
the wider value to a narrower "I32 count"