linux-user: create a dummy per arch cpu_loop.c

linux-user: create a dummy per arch cpu_loop.c

Create a cpu_loop-common.h for future use by
these new files and use it in the existing
main.c

Introduce target_cpu_copy_regs():
declare the function in cpu_loop-common.h
and an empty function for each target,
to move all the cpu_loop prologues to this function.

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20180411185651.21351-2-laurent@vivier.eu>