test: Disable vector reduce operation on mips64.

#21091

Author: alexrp

test/behavior/vector.zig

@@ -773,6 +773,7 @@ test "vector reduce operation" {
     if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
     if (builtin.zig_backend == .stage2_c and comptime builtin.cpu.arch.isArmOrThumb()) return error.SkipZigTest;
     if (builtin.zig_backend == .stage2_riscv64) return error.SkipZigTest;
+    if (builtin.cpu.arch.isMIPS64()) return error.SkipZigTest; // https://github.com/ziglang/zig/issues/21091
 
     const S = struct {
         fn testReduce(comptime op: std.builtin.ReduceOp, x: anytype, expected: anytype) !void {