Perl_sv_vcatpvfn_flags: move some macros earlier

Perl_sv_vcatpvfn_flags: move some macros earlier

There are some macro definitions in the body of Perl_sv_vcatpvfn_flags()
which handle some possible differences between double and long double.
Move these to before the function as they will shortly need to be visible
to a new helper function. At the same time, prefix their names with with
VCATPVFN_ to make clear what they're for.

For the same reason I've also added a new typedef, vcatpvfn_long_double_t.

I also eliminated the FV_ISFINITE macro definition as its no longer used.