cpus: Move CPU code from exec.c to cpus-common.c

This code was introduced with SMP support in commit 6a00d60127,
later commit 267f685b8b moved CPU list management to common code
but forgot this code. Move now and simplify ifdef'ry.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200702104017.14057-1-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>