ia64: move nr-irqs.h to include/generated

Avoid generating files in the now deprecated asm-ia64 dir
Simplified the logic in the Makefile when editing stuff in the area

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>