Marc-Aurel Zent (@mzent) commented about include/winternl.h:
NTSYSAPI NTSTATUS WINAPI NtFsControlFile(HANDLE,HANDLE,PIO_APC_ROUTINE,PVOID,PIO_STATUS_BLOCK,ULONG,PVOID,ULONG,PVOID,ULONG); NTSYSAPI NTSTATUS WINAPI NtGetContextThread(HANDLE,CONTEXT*); NTSYSAPI ULONG WINAPI NtGetCurrentProcessorNumber(void); +NTSYSAPI ULONG WINAPI NtGetCurrentProcessorNumberEx(PROCESSOR_NUMBER*);
FWIW `NtGetCurrentProcessorNumberEx()` is also a syscall.