(x86/Sparc/PPC)-user: fix cpu_copy

b55a37c981914aa8ecd21b9a2a2fb37f39b917c5 moved the call to cpu_reset
to user emulators. But cpu_copy also initializes a CPU structure, so add the
call also there.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
