Create real debug package

Previously, using debug option for a linux kernel package resulted in the creation of empty debug packages. This was because in PKGBUILD we do strip manually, without using the corresponding makepkg option. I assume this was done before because makepkg was doing strip wrongly for the vmlinux file, since it erroneously did strip for shared binaries for all statically linked files. To solve this problem, I proposed a fix in pacman that hit in the 7.0.0 release (pacman/pacman!200 (merged)) (maybe we should wait a while for the release to settle down?), which should solve the strip vmlinux problems as well, thus:

1. We don't need to do manual strip anymore and we can put this job on makepkg, as it should be.

2. Point 1 ensures that debug packages are no longer empty and makepkg will handle them properly for the kernel.

Creating debug packages for kernel can be extremely useful for unrolling kernel dumps obtained after a kernel panic using kdump. These packages are already provided in distributions like Debian/Ubuntu/Fedora, so I think it would be a good idea for Arch to join the list :)

Fixes: #2

P.S. I haven't bumped pkgrel because as I noticed the maintainers prefer to do it themselves, but if I need to do it, let me know.