linux-user support for target/xtensa port of QEMU

Issues

[+] preadv/pwritev don't translate between target low/high parts of file offset and host low/high

[+] clock_settime is not supported

[-] FUTEX_UNLOCK_PI and other -PI primitives are not implemented

[-] vfork is not supported

[-] clone flags, such as custom signal on child termination, are not supported