std: re-enable auto hash test

It was temporarily disabled due to a self-hosted compiler
miscompilation.

closes #12178

lib/std/hash/auto_hash.zig

@@ -374,12 +374,6 @@ test "testHash struct" {
 }
 
 test "testHash union" {
-    const builtin = @import("builtin");
-    if (builtin.zig_backend == .stage2_llvm and builtin.mode == .ReleaseSafe) {
-        // https://github.com/ziglang/zig/issues/12178
-        return error.SkipZigTest;
-    }
-
     const Foo = union(enum) {
         A: u32,
         B: bool,