libio: Set _vtable_offset before calling _IO_link_in [BZ #32148]

libio: Set _vtable_offset before calling _IO_link_in [BZ #32148]

Since _IO_vtable_offset is used to detect the old binaries, set it
in _IO_old_file_init_internal before calling _IO_link_in which checks
_IO_vtable_offset.  Add a glibc 2.0 test with copy relocation on
_IO_stderr_@GLIBC_2.0 to verify that fopen won't cause memory corruption.
This fixes BZ #32148.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>