[ping] make __sf_fake_std{in,out,err} weak in impure.c

[Kazu Hirata]

Hi,

Could somebody review the following patch?

  PATCH: make __sf_fake_std{in,out,err} weak in impure.c
  http://sourceware.org/ml/newlib/2007/msg00037.html

Thanks,

Kazu Hirata