cpu/arm/k60: Add _isatty() definition.

If newlib is built without REENTRANT_SYSCALLS_PROVIDED, then isatty()
is defined to call _isatty, not _isatty_r.

newlib/newlib/libc/syscalls/sysisatty.c:

    int
    _DEFUN (isatty, (fd),
         int fd)
    {
    #ifdef REENTRANT_SYSCALLS_PROVIDED
      return _isatty_r (_REENT, fd);
    #else
      return _isatty (fd);
    #endif
    }

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>