libstdc++/5820: EOF not handled correctly on input

[Benjamin Kosnik]

See this bit
http://gcc.gnu.org/ml/libstdc++/2002-04/msg00024.html

In particular, _GLIBCPP_AVOID_FSEEK defines in 
config/os/gnu/bits/os_defines.h should do it. Can you try this please?

-benjamin