Perl_sv_vcatpvfn_flags: handle mixed utf8 better

Perl_sv_vcatpvfn_flags: handle mixed utf8 better

Once the output string gets upgraded to utf8 (e.g. due to a utf8 %s
argument), any remaining appending of plain (non-%) parts of the
format string becomes very inefficient. It basically creates an
SV out of the next format chunk, upgrades that SV to utf8, then
appends the upgraded buffer.

This commits makes it just append the format chunk byte by byte, upgrading
in the fly if that byte is !NATIVE_BYTE_IS_INVARIANT