Make assignment over glob copies much faster

Make assignment over glob copies much faster

sv_force_normal is passed the SV_COW_DROP_PV flag if the scalar is
about to be written over.  That flag is not currently used.  We can
speed up assignment over fake GVs a lot by taking advantage of the flag.

Before and after:

$ time ./perl -e '$x = *foo, undef $x for 1..2000000'

real	0m4.264s
user	0m4.248s
sys	0m0.007s
$ time ./perl -e '$x = *foo, undef $x for 1..2000000'

real	0m1.820s
user	0m1.812s
sys	0m0.005s