linux-user: move socket.h generic definitions to generic/sockbits.h

and include the file from architectures without specific definitions

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20180519092956.15134-5-laurent@vivier.eu>
diff --git a/linux-user/x86_64/sockbits.h b/linux-user/x86_64/sockbits.h
new file mode 100644
index 0000000..0e4c8f0
--- /dev/null
+++ b/linux-user/x86_64/sockbits.h
@@ -0,0 +1 @@
+#include "../generic/sockbits.h"