fix: enable llvm by default for debug builds

The native backends for zig are still very slow. Enable llvm by default so debug
builds are generally useable. For quick builds, explicitly disable llvm with
`-Duse_llvm=false`.

build.zig

@@ -582,6 +582,9 @@ pub fn build_exe(
     if (use_llvm) |value| {
         exe.use_llvm = value;
         exe.use_lld = value;
+    } else {
+        exe.use_llvm = true;
+        exe.use_lld = true;
     }
     if (pie) |value| exe.pie = value;
     exe.root_module.addImport("build_options", options_mod);