std: Re-enable union behaviour test for mips targets

test/stage1/behavior/union.zig

@@ -623,9 +623,6 @@ test "0-sized extern union definition" {
 }
 
 test "union initializer generates padding only if needed" {
-    // https://github.com/ziglang/zig/issues/5127
-    if (std.Target.current.cpu.arch == .mips) return error.SkipZigTest;
-
     const U = union(enum) {
         A: u24,
     };