pp_sort.c: move the definitions of comparators

This is a workaround for a bug in GCC 12:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107931

For some reason, when compiling with the -Og flag, GCC 12 is unable to
inline the comparators. It's fatal because they're marked with
__attribute__((always_inline)).

To avoid the bug, the comparators have to be defined before their
first use.

Fixes Perl#19776, fixes Perl#20395