llvm: Use unreachable in targetTriple() for targets without LLVM support.

src/codegen/llvm.zig

@@ -81,12 +81,14 @@ pub fn targetTriple(allocator: Allocator, target: std.Target) ![]const u8 {
         .spirv => "spirv",
         .spirv32 => "spirv32",
         .spirv64 => "spirv64",
-        .kalimba => "kalimba",
         .lanai => "lanai",
         .wasm32 => "wasm32",
         .wasm64 => "wasm64",
         .ve => "ve",
-        .spu_2 => return error.@"LLVM backend does not support SPU Mark II",
+
+        .kalimba,
+        .spu_2,
+        => unreachable, // Gated by hasLlvmSupport().
     };
     try llvm_triple.appendSlice(llvm_arch);
     try llvm_triple.appendSlice("-unknown-");