x86/entry/64: Add __SYSCALL_COMMON()

Add a __SYSCALL_COMMON() macro to the syscall table, which simplifies syscalltbl.sh.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200313195144.164260-12-brgerst@gmail.com