module, bpf: Store BTF base pointer in struct module

...as this will allow split BTF modules with a base BTF
representation (rather than the full vmlinux BTF at time of
BTF encoding) to resolve their references to kernel types in a
way that is more resilient to small changes in kernel types.

This will allow modules that are not built every time the kernel
is to provide more resilient BTF, rather than have it invalidated
every time BTF ids for core kernel types change.

Fields are ordered to avoid holes in struct module.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240620091733.1967885-3-alan.maguire@oracle.com