linux-user: Enable NPTL for OpenRISC

linux-user: Enable NPTL for OpenRISC

The OpenRISC kernel ignores CLONE_SETTLS in its copy_thread()
implementation, so a cpu_set_tls() implementation is a no-op.
cpu_clone_regs() was setting the syscall return value in the
wrong register -- it is gpr[11], not gpr[2]. With these two
things fixed, we can compile with NPTL enabled.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Jia Liu <proljc@gmail.com>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>