add S_sv_catpvn_simple() for use by sprintf

add S_sv_catpvn_simple() for use by sprintf

Currently Perl_sv_vcatpvfn_flags() uses an unrolled sv_catpvn_nomg()
to append floating point formats, a call to sv_catpvn_nomg() to append
non-% parts of the format, and a few other non-performance-critical
calls to sv_catpvn_nomg().

Move the unrolled code block into an inline static function, and make
the non-% appending use it too.