We can avoid the need to explicitly exclude it later if we just put it
somewhere else to begin with.

linux is unusual in that we include its sources in an output.  There's
no point unpacking into /build when we're going to copy the sources
into $dev later.  Let's unpack directly into the final destination of
the code, and save copying a whole kernel source tree (often across
filesystems!).

This also means that Kbuild knows the location of the sources, which
will allow us to install the GDB scripts — some scripts are generated,
and some are not, so the generated ones end up in the build directory,
accompanied by symlinks to the non-generated ones in the source
directory.

We've basically been reimplementing this — by default it contains
vmlinux, dtbs (on applicable architectures), modules, and architecture
specific stuff like $(KBUILD_IMAGE) and a couple of other
miscellaneous files.

These are required to debug kernel modules.  Since we're now able to
do that, there's another reason besides BTF to enable DEBUG_INFO, so
I've done that for pre-BTF kernel modules as well here.

For GDB to get configured correctly, vmlinux-gdb.py has to be two
directories up from scripts/gdb, and vmlinux has to be next to
vmlinux-gdb.py.  The least invasive way to satisfy these constraints
is to make vmlinux a symlink, which GDB will resolve before looking
for vmlinux-gdb.py.

Tested both ways of getting the scripts into GDB that I know of:

gdb /nix/store/7n77ijlxkxr6d613h02lr707kvjx6j1k-linux-6.1.19-dev/vmlinux \
    -iex 'add-auto-load-safe-path /nix/store/7n77ijlxkxr6d613h02lr707kvjx6j1k-linux-6.1.19-dev/lib/modules/6.1.19/build/vmlinux-gdb.py' \
    -ex 'lx-version' \
    -ex 'q'
gdb /nix/store/7n77ijlxkxr6d613h02lr707kvjx6j1k-linux-6.1.19-dev/vmlinux \
    -ex 'source /nix/store/7n77ijlxkxr6d613h02lr707kvjx6j1k-linux-6.1.19-dev/lib/modules/6.1.19/build/vmlinux-gdb.py' \
    -ex 'lx-version' \
    -ex 'q'

Also tested that the strip changes don't result in meaningful output
size changes (there's some small variation due to BTF data not always
coming out the same size, which is unrelated), and built every kernel
I can on x86_64 to make sure I'm not relying on build system behaviour
specific to newer kernels.