linux-user: fix broken cpu_copy()

New threads always point at the same env which is incorrect and usually
leads to a crash.

Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>