kbuild: add debug package to pacman PKGBUILD

Add a new debug package to the PKGBUILD for the pacman-pkg target. The
debug package includes the non-stripped vmlinux file with debug symbols
for kernel debugging and profiling. The file is installed at
/usr/src/debug/${pkgbase}, with a symbolic link at
/usr/lib/modules/$(uname -r)/build/vmlinux. The debug package is built
by default.

Signed-off-by: Jose Fernandez <jose.fernandez@linux.dev>
Reviewed-by: Peter Jung <ptr1337@cachyos.org>
Acked-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>