lib: libc: arcmwdt: replace the fileno macro

The fileno macro defined in ARC MWDT headers conflicts with the fileno()
function defined in lib/posix/options/device_io.c. We should undefine it
and replace with a POSIX-compliant declaration and a weak definition.

Signed-off-by: Ilya Tagunov <Ilya.Tagunov@synopsys.com>