C: fix utf8 precomp len (re-utf8)

we do not have Encode nor bytes, so use the pv trick used before
to get the bytes length of an utf8 PMOP qr string.
Detected on 5.20
C: >=5.18 init utf8_tofold swash dynamically to avoid free on static errors

We need this swash with utf8 m//i, but since the &PL_utf8_tofold is empty
we need to init it somehow, to avoid free static pv2... errors (the old swash).
This does not save the folding tables into the binary.
We really need to init utf8_tofold statically (TODO)