sv.c: make PUSH_EXTEND_MORTAL__SV_C an inline function

[mauke]

This macro could easily be a function, so why not make it one?

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• This set of changes does not require a perldelta entry.

[richardleach]

When doing something like sv_2mortal(newSViv(27)), the checks in Perl_sv_2mortal for (!sv) and SvIMMORTAL(sv) are unnecessary and what remains is this new function's body plus SvTEMP_on(sv).

Is it worth making this function Perl_push_extend_mortal and part of the API, whether that's in sv.c or in sv_inline.h?