Remove usage of unstable proc macro features

[nspin]

Remove usage of the following unstable features:

#![feature(proc_macro_hygiene)]
#![feature(stmt_expr_attributes)]

As a consequence, we can no longer use the sel4_config::* attribute macros on statements of non-inline module declarations.

So, we replace #![sel4_config::sel4_cfg] with sel4_config::sel4_cfg_if! {} in the relevant cases, and add a new expression macro called sel4_config::sel4_cfg_wrap_match! {} to be used in place of #[sel4_config::sel4_cfg_match] until those unstable features are stabilized.

In the cases relevant to this PR, attribute macros are generally preferable over expression macros because the play more nicely with external tools like rustfmt, but, for now, we must make due with expression macros.