This makes __NR_kexec_file_load available on architectures where the kernel UAPI headers don't define it, matching the runtime fallback path in src/libc/kexec.c which is gated on #ifdef __NR_kexec_file_load.
This makes __NR_kexec_file_load available on architectures where the kernel UAPI headers don't define it, matching the runtime fallback path in src/libc/kexec.c which is gated on #ifdef __NR_kexec_file_load.