sv.h: Turn off AMAGIC flag in Gv_AMG
This makes no functional changes.
If a stash has its AMAGIC flag on, then when the overload caches are
updated we can turn off the flag if it turns out that there is no
overloading.
Gv_AMG is the easiest place to unset this flag, as Gv_AMupdate has
‘return 0’ in more than one place.
This is for efficiency’s sake, as an object that inherits overloading
but then loses it through @ISA changes will still have to go through
extra checks due to SvAMAGIC returning true.
This also offsets an inefficiency to be introduced in later commits:
changes to @ISA will set the AMAGIC flag.