[perl #117855] Store CopFILEGV in a pad under ithreads

[perl #117855] Store CopFILEGV in a pad under ithreads

This saves having to allocate a separate string buffer for every cop
(control op; every statement has one).

Under non-threaded builds, every cop has a pointer to the GV for that
source file, namely *{"_<filename"}.

Under threaded builds, the name of the GV used to be stored instead.

Now we store an offset into the per-interpreter PL_filegvpad, which
points to the GV.

This makes no significant speed difference, but it reduces mem-
ory usage.