pp_sys.c: Simplify uses of sv_len_utf8

pp_sys.c: Simplify uses of sv_len_utf8

sv_len_utf8 is now careful not to record caches on magical or over-
loaded scalars (any non-PV, in fact).  It also returns the number of logical characters correctly, regardless of whether its input is utf8.
So we can take advantage of that to simplify pp_sysread.

For pp_syswrite, we can use sv_or_pv_len_utf8 with the existing
string buffer.