linux-user: fix the breakpoint inheritance in spawned threads

When a thread is spawned, cpu_copy re-initializes
the bp & wp lists of current thread, instead of the ones
of the new thread.
The effect is that breakpoints are no longer hit.

Signed-off-by: Thierry Bultel <thierry.bultel@basystemes.fr>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
