x86/entry/64: Move sys_ni_syscall stub to common.c

so it can be available to multiple syscall tables.  Also directly return
-ENOSYS instead of bouncing to the generic sys_ni_syscall().

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200313195144.164260-7-brgerst@gmail.com