lib: libc: arcmwdt: Fix userspace write() and read() mishap

The newlib `write()` and `read()` functions must call the
`z_zephyr_write_stdout()` and `z_zephyr_read_stdin()` syscall functions
in order to function properly in a user mode context.

The existing incorrect implementation was copied off the newlib hooks
implementation, which was corrected in the previous commit.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>