x86/entry: Refactor SYSCALL_DEFINEx macros

Pull the common code out from the SYSCALL_DEFINEx macros into a new
__SYS_STUBx macro.  Also conditionalize the X64 version in preparation for
enabling syscall wrappers on 32-bit native kernels.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
Reviewed-by: Andy Lutomirski <luto@kernel.org>
Link: https://lkml.kernel.org/r/20200313195144.164260-2-brgerst@gmail.com