default to CORE::warn in $SIG{__WARN__} handlers

default to CORE::warn in $SIG{__WARN__} handlers

As with CORE::die and $SIG{__DIE__}, it turns out CORE::warn is
safe to use inside $SIG{__WARN__} handlers without triggering
infinite recursion.  So fall back to reusing CORE::warn instead
of creating a new sub.