doc/go1.21: add release notes for linux/loong64

Fixes #53301

Change-Id: Id447d57d43b12c3748267295928d45a089549340
Reviewed-on: https://go-review.googlesource.com/c/go/+/507815
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Bypass: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
TryBot-Bypass: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

doc/go1.21.html

@@ -1244,6 +1244,13 @@ <h3 id="PPC64">ppc64/ppc64le</h3>
   <code>c-archive</code>, <code>c-shared</code>, <code>shared</code>, <code>pie</code>, <code>plugin</code>.
 </p>
 
+<h3 id="loong64">loong64</h3>
+
+<p><!-- go.dev/issue/53301, CL 455075, CL 425474, CL 425476, CL 425478, CL 489576 -->
+  The <code>linux/loong64</code> port now supports <code>-buildmode=c-archive</code>,
+  <code>-buildmode=c-shared</code> and <code>-buildmode=pie</code>.
+</p>
+
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