libio: Do not autogenerate stdio_lim.h

Instead define the required fields in system dependend files.  The only
system dependent definition is FILENAME_MAX, which should match POSIX
PATH_MAX, and it is obtained from either kernel UAPI or mach headers.
Currently set pre-defined value from current kernels.

It avoids a circular dependendy when including stdio.h in
gen-as-const-headers files.

Checked on x86_64-linux-gnu and i686-linux-gnu
Reviewed-by: Carlos O'Donell <carlos@redhat.com>