FROMLIST: rust: generate a fatal error if BINDGEN_TARGET is undefined

Generate a friendly fatal error if the target triplet is undefined for
bindgen, rather than having the compiler generate obscure error messages
during the build stage.

This piece of code is copied from `scripts/Makefile.clang`.

Before this commit, error messages might look like:

error: unknown argument: '-mno-riscv-attribute'
error: unsupported argument 'medany' to option '-mcmodel=' for target
'unknown'
error: unsupported option '-march=' for target ''
error: unsupported option '-mno-save-restore' for target ''
error: unknown target triple 'unknown'
panicked at bindgen/ir/context.rs:562:15:
libclang error; possible causes include:
- Invalid flag syntax
- Unrecognized flags
- Invalid flag arguments
- File I/O errors
- Host vs. target architecture mismatch

Acked-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Asuna Yang <SpriteOvO@gmail.com>
Link: https://lore.kernel.org/r/20251204-gcc-rust-v5-v5-2-2d4f20d86c24@gmail.com
Signed-off-by: Han Gao <gaohan@iscas.ac.cn>
Signed-off-by: Mingcong Bai <jeffbai@aosc.io>

Author: SpriteOvO

scripts/Makefile.rust

@@ -6,4 +6,8 @@ BINDGEN_TARGET_loongarch	:= loongarch64-linux-gnusf
 BINDGEN_TARGET_um	:= $(BINDGEN_TARGET_$(SUBARCH))
 BINDGEN_TARGET		:= $(BINDGEN_TARGET_$(SRCARCH))
 
+ifeq ($(BINDGEN_TARGET),)
+$(error add '--target=' option to scripts/Makefile.rust)
+else
 export BINDGEN_TARGET
+endif