rust: kbuild: introduce `core-flags` and `core-skip_flags`

In the next commits we are introducing `*-{cfgs,skip_flags,flags}`
variables for other crates.

Thus do so here for `core`, which simplifies a bit the `Makefile`
(including the next commit) and makes it more consistent.

This means we stop passing `-Wrustdoc::unescaped_backticks` to `rustc`
and `-Wunreachable_pub` to `rustdoc`, i.e. we skip more, which is fine
since it shouldn't have an effect.

In addition, use `:=` for `core-cfgs` to make it consistent with the
upcoming additions.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Tested-by: Gary Guo <gary@garyguo.net>
Tested-by: Jesung Yang <y.j3ms.n@gmail.com>
Link: https://patch.msgid.link/20251124151837.2184382-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

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