ARM: add kexec_file_load system call number

A couple of architectures including arm64 already implement the
kexec_file_load system call, on many others we have assigned a system
call number for it, but not implemented it yet.

Adding the number in arch/arm/ lets us use the system call on arm64
systems in compat mode, and also reduces the number of differences
between architectures. If we want to implement kexec_file_load on ARM
in the future, the number assignment means that kexec tools can already
be built with the now current set of kernel headers.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>