sv.c: Allow COWs through sv_magic unscathed

sv.c: Allow COWs through sv_magic unscathed

There is no need to run COWs through sv_force_normal when
attaching magic to them via sv_magic.  (I don’t know about
PERL_OLD_COPY_ON_WRITE, but I left it in, just to be sure.)

Before this, only sv_magicext could attach magic to SVs that were
already COWs.  That meant pos($cow)=7 would leave it as a COW, but
copy-on-write would never happen with tainted strings.