avoid more vivification in Carp

avoid more vivification in Carp

Avoid vivifying the overload::StrVal subroutine, its glob, or its stash.
This is done in the same way as the existing avoidance of vivification of
utf8::is_utf8 and utf8::downgrade.  However, the check has to be made at
runtime, whereas the utf8-related ones are checked at load time, because
the utf8 ones are built into the perl core (only absent pre perl 5.8)
but overload is a separate module that can be loaded later.