Remove references to old mailing lists that have moved to discourse. Replace with links to discourse.

[tonic]

I think these changes are pretty straight forward. If anyone wants to give the thumbs up, please do so. You don't need to be owners of the related websites/docs.

compiler-rt/www/index.html

<div id="content">
   <p>Tests for sanitizer runtimes are ported to
   <a href="https://llvm.org/docs/CommandGuide/lit.html">llvm-lit</a> and are
   run by <b>make check-all</b> command in LLVM/Clang/compiler-rt build tree.</p>
 
   <p>compiler-rt libraries are installed to the system with <b>make install</b>
   command in either LLVM/Clang/compiler-rt or standalone
   compiler-rt build tree.</p>
 
-  <p>compiler-rt doesn't have its own mailing list, if you have questions please
-     email the <a
-    href="https://lists.llvm.org/mailman/listinfo/llvm-dev">llvm-dev</a> mailing
-    list.  Commits to compiler-rt are automatically sent to the
-    <a
+  <p>If you have questions, please ask on the <a href="https://discourse.llvm.org/c/runtimes/64">
+   Discourse forums</a> under the Runtime category.  Commits to compiler-rt are automatically 
+   sent to the <a
   href="https://lists.llvm.org/mailman/listinfo/llvm-commits">llvm-commits</a>
     mailing list.</p>
 </div>
 </body>
 </html>

libclc/www/index.html

 libclc currently supports the AMDGCN, and R600 and NVPTX targets, but
 support for more targets is welcome.
 </p>
 
 <h2>Download</h2>
 
 <tt>git clone https://github.com/llvm/llvm-project.git</tt> (<a href="https://github.com/llvm/llvm-project/tree/libclc">View sources</a>)
 
-<h2>Mailing List</h2>
+<h2>Discourse forums</h2>
 
[e-mail] (<a href="https://lists.llvm.org/cgi-bin/mailman/listinfo/libclc-dev">subscribe/unsubscribe</a>, <a href="https://lists.llvm.org/pipermail/libclc-dev/">archives</a>)
+<a href="https://discourse.llvm.org/c/runtimes/opencl/65">Runtimes - OpenCL category</a>
 
 </body>
 </html>

libcxxabi/www/index.html

<div id="menu">
   <div class="submenu">
     <label>libc++abi Info</label>
     <a href="/index.html">About</a>
   </div>
 
   <div class="submenu">
     <label>Quick Links</label>
     <a href="https://libcxx.llvm.org/">libc++</a>
-    <a href="https://lists.llvm.org/mailman/listinfo/libcxx-dev">libcxx-dev</a>
+    <a href="https://discourse.llvm.org/c/runtimes/libcxx/10">Discourse Forums</a>
     <a href="https://lists.llvm.org/mailman/listinfo/libcxx-commits">libcxx-commits</a>
     <a href="https://bugs.llvm.org/">Bug Reports</a>
     <a href="https://github.com/llvm/llvm-project/tree/main/libcxxabi/">Browse Sources</a>
   </div>
 </div>
 
 <div id="content">
   <!--*********************************************************************-->