kernel: include: generate headers in parallel

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
kernel: include: fix dependency on regen script and input files

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
kernel: include: regen linux_syscall_numbers.h

Signed-off-by: Mike Frysinger <vapier@gentoo.org>