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>

linux-user/Makefile.objs

@@ -1,6 +1,7 @@
 obj-y = main.o syscall.o strace.o mmap.o signal.o \
 	elfload.o linuxload.o uaccess.o uname.o \
-	safe-syscall.o $(TARGET_ABI_DIR)/signal.o
+	safe-syscall.o $(TARGET_ABI_DIR)/signal.o \
+        $(TARGET_ABI_DIR)/cpu_loop.o
 
 obj-$(TARGET_HAS_BFLT) += flatload.o
 obj-$(TARGET_I386) += vm86.o