x86: fix smp_stacktop on 32-bit

smp_stacktop in 32-bit is fixed to some magic address. Use the address
of the memory that was reserved for the stack instead.

Signed-off-by: Nadav Amit <namit@vmware.com>
Message-Id: <20200624203602.44659-1-namit@vmware.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>