FIONREAD and IRIX 6

[Richard Sandiford]

There are 4 27_io/basic_filebuf/sgetn/char failures on IRIX:

   http://gcc.gnu.org/ml/gcc-testresults/2004-01/msg01339.html

These are due to IRIX's FIONREAD taking a "size_t *" rather than "int *"
argument.  __basic_file<char>::showmanyc() ends up returning the high word
of the size_t, which is 0 for these tests.

What's the preferred way of handling this sort thing?  Is it OK to
add a new #define to config/os/irix/irix6.5/os_defines.h and use
that to alter the type in showmanyc()?  If so, what sort of name
would you suggest?

Richard