[PATCH v5 2/4] rust: generate a fatal error if BINDGEN_TARGET is undefined

[Asuna Yang]

I noticed that RISC-V CI bjorn/dtb-warn-rv64 [^1] is failing somehow due 
to the error here. I'll look into it tomorrow.

[^1]: 
https://patchwork.kernel.org/project/linux-riscv/patch/20251204-gcc-rust-v5-v5-4-2d4f20d86c24@gmail.com/