Allowlist expected cfgs

See https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg.

ebpf/aya-ebpf-bindings/build.rs

@@ -9,4 +9,5 @@ fn main() {
         let arch = arch.split_once('-').map_or(&*arch, |x| x.0);
         println!("cargo:rustc-cfg=bpf_target_arch=\"{arch}\"");
     }
+    println!("cargo::rustc-check-cfg=cfg(bpf_target_arch, values(\"x86_64\",\"arm\",\"aarch64\",\"riscv64\"))");
 }

ebpf/aya-ebpf-cty/build.rs

@@ -9,4 +9,5 @@ fn main() {
         let arch = arch.split_once('-').map_or(&*arch, |x| x.0);
         println!("cargo:rustc-cfg=bpf_target_arch=\"{arch}\"");
     }
+    println!("cargo::rustc-check-cfg=cfg(target_arch, values(\"asmjs\",\"nvptx\",\"xtensa\"))");
 }

ebpf/aya-ebpf/build.rs

@@ -10,6 +10,8 @@ fn main() {
         let arch = arch.split_once('-').map_or(&*arch, |x| x.0);
         println!("cargo:rustc-cfg=bpf_target_arch=\"{arch}\"");
     }
+    println!("cargo::rustc-check-cfg=cfg(bpf_target_arch, values(\"x86_64\",\"arm\",\"aarch64\",\"riscv64\"))");
+    println!("cargo::rustc-check-cfg=cfg(unstable)");
 }
 
 #[rustversion::nightly]