softmmu: List CPU types again

Commit e0220bb5 made cpus.c target-agnostic but didn't notice
the cpu_list() function is only defined in target-specific code
in "cpu.h". Move list_cpus() declaration to "exec/cpu-common.h"
because this function is not softmmu-specific and can also be
used by user-mode, along with moving its implementation to cpu.c,
which is compiled per target.

Fixes: e0220bb5 ("softmmu: Build target-agnostic objects once")
Reported-by: Max Filippov <jcmvbkbc@gmail.com>
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220314140108.26222-1-philippe.mathieu.daude@gmail.com>
Tested-by: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>