riscv: disable failing test

Author: Rexicon226

test/behavior/optional.zig

@@ -632,6 +632,7 @@ test "result location initialization of optional with OPV payload" {
     if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_riscv64) return error.SkipZigTest;
 
     const S = struct {
         x: u0,