linux-user: Introduce cpu_clone_regs_parent

linux-user: Introduce cpu_clone_regs_parent

We will need a target-specific hook for adjusting registers
in the parent during clone.  Add an empty inline function for
each target, and invoke it from the proper places.

Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20191106113318.10226-11-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>