x86-64: Wire up getcpu syscall

getcpu is available as a vdso entry and an emulated vsyscall.
Programs that for some reason don't want to use the vdso should
still be able to call getcpu without relying on the slow emulated
vsyscall.  It costs almost nothing to expose it as a real syscall.

We also need this for the following patch in vsyscall=native mode.
Signed-off-by: NAndy Lutomirski <luto@mit.edu>
Link: http://lkml.kernel.org/r/6b19f55bdb06a0c32c2fa6dba9b6f222e1fde999.1312988155.git.luto@mit.eduSigned-off-by: NH. Peter Anvin <hpa@linux.intel.com>

arch/x86/include/asm/unistd_64.h

@@ -681,6 +681,8 @@ __SYSCALL(__NR_syncfs, sys_syncfs)
 __SYSCALL(__NR_sendmmsg, sys_sendmmsg)
 #define __NR_setns				308
 __SYSCALL(__NR_setns, sys_setns)
+#define __NR_getcpu				309
+__SYSCALL(__NR_getcpu, sys_getcpu)
 
 #ifndef __NO_STUBS
 #define __ARCH_WANT_OLD_READDIR