sh: add __NR_syscalls along with NR_syscalls

NR_syscalls macro holds the number of system call exist
in sh architecture. We have to change the value of NR-
_syscalls, if we add or delete a system call.

One of the patch in this patch series has a script which
will generate a uapi header based on syscall.tbl file.
The syscall.tbl file contains the total number of system
calls information. So we have two option to update NR_sy-
scalls value.

1. Update NR_syscalls in asm/unistd.h manually by count-
   ing the no.of system calls. No need to update NR_sys-
   calls until we either add a new system call or delete
   existing system call.

2. We can keep this feature it above mentioned script,
   that will count the number of syscalls and keep it in
   a generated file. In this case we don't need to expli-
   citly update NR_syscalls in asm/unistd.h file.

The 2nd option will be the recommended one. For that, I
added the __NR_syscalls macro in uapi/asm/unistd_32/64.h
along with NR_syscalls which is moved to asm/unistd.h.
The macro __NR_syscalls also added for making the name
convention same across all architecture. While __NR_sys-
calls isn't strictly part of the uapi, having it as part
of the generated header to simplifies the implementation.
We also need to enclose this macro with #ifdef __KERNEL__
to avoid side effects.

Signed-off-by: Firoz Khan <firoz.khan@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>