libio vtable verification failures with multiple libcs

[Florian Weimer]

This is just a heads-up that the Ruby test suite loads libc.so.6 with an 
absolute path, bypassing AT_PLATFORM directories.  This can lead to 
multiple libc.so.6 copies within the same process:

   <https://bugzilla.redhat.com/show_bug.cgi?id=1361037#c17>

There is a test case which creates a FILE * object using one libc and 
passes it to fprintf in another libc, which fails vtable verification 
because the vtable is not known to fprintf.  The dlmopen bypass for 
verification does not kick in because dlmopen is not used.

I don't think there is anything we should change on the glibc side as 
far as libio is concerned.  Fortunately, this is just a Ruby test case 
for the Fiddle FFI module.  Correct Fiddle use should be unaffected.

(But I do wonder if we should detect that the libc.so.6 soname is loaded 
a second time and fail.)

Florian