test: skip alternative constraints behavior test on LoongArch

llvm/llvm-project#159200

Author: alexrp

test/behavior/asm.zig

@@ -67,6 +67,7 @@ test "alternative constraints" {
     if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_spirv) return error.SkipZigTest;
     if (builtin.zig_backend == .stage2_riscv64) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_llvm and builtin.cpu.arch.isLoongArch()) return error.SkipZigTest; // https://github.com/llvm/llvm-project/issues/159200
 
     if (builtin.zig_backend == .stage2_c and builtin.os.tag == .windows) return error.SkipZigTest; // MSVC doesn't support inline assembly