Add CopFILEAVn() and use it when cleaning up COP pointers

Add CopFILEAVn() and use it when cleaning up COP pointers

On threaded builds CopFILEAV() calls gv_fetchfile(), which always
created the *{"::_<filenamehere"} glob, so the attempted clean up
here could recreate the glob, even if it has already been removed
when cleaning up a string eval.

To avoid this, add CopFILEAVn() that never creates the glob,
nor the AV so that the clean up never adds new objects.

This change makes the check for PL_phase unnecessary, but that check
is much cheaper than the call for gv_fetchfile_flags() that the
macro hides, so retain the check.