__libc_stack_end is broken?

[Michael Casadevall]

While fixing Ruby on Hurd, I discovered  __libc_stack_end is (seemingly)
broken. It seems on Linux, this points to the end the stack for the
process, but on Hurd this is broken (it points at an invalid location
outside of a processes address space). 

A quick gogle says though that this symbol shouldn't even be exported,
so I'm not sure what the proper behavior in this regard is. Any comments
would be appericated because it will also influence how the ruby patch
is done.
Michael