disable failing @mulAdd behavior test for aarch64-macos

See #9900

test/behavior/muladd.zig

@@ -1,3 +1,4 @@
+const builtin = @import("builtin");
 const expect = @import("std").testing.expect;
 
 test "@mulAdd" {
@@ -24,6 +25,10 @@ fn testMulAdd() !void {
         var c: f64 = 6.25;
         try expect(@mulAdd(f64, a, b, c) == 20);
     }
+    if (builtin.os.tag == .macos and builtin.cpu.arch == .aarch64) {
+        // https://github.com/ziglang/zig/issues/9900
+        return error.SkipZigTest;
+    }
     {
         var a: f16 = 5.5;
         var b: f128 = 2.5;