kexec: new syscall kexec_file_load() declaration

kexec: new syscall kexec_file_load() declaration

This is the new syscall kexec_file_load() declaration/interface.  I have
reserved the syscall number only for x86_64 so far.  Other architectures
(including i386) can reserve syscall number when they enable the support
for this new syscall.

Signed-off-by: Vivek Goyal <[email protected]>
Cc: Michael Kerrisk <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Yinghai Lu <[email protected]>
Cc: Eric Biederman <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Matthew Garrett <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Dave Young <[email protected]>
Cc: WANG Chao <[email protected]>
Cc: Baoquan He <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>