kbuild: complain about missing system calls

Most system calls seems to get added to i386 first. This patch
automatically generates a warning for any new system call which is
implemented on i386 but not the architecture currently being compiled.
On PowerPC at the moment, for example, it results in these warnings:
init/missing_syscalls.h:935:3: warning: #warning syscall sync_file_range not implemented
init/missing_syscalls.h:947:3: warning: #warning syscall getcpu not implemented
init/missing_syscalls.h:950:3: warning: #warning syscall epoll_pwait not implemented

The file scripts/checksyscalls.sh list a number of legacy system calls
that are ignored because they only makes sense on i386 systems.

Other contributors to this patch are Russell King <rmk+lkml@arm.linux.org.uk>
and Stéphane Jourdois <kwisatz@rubis.org>

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>