doc: add riscv64 to ports list

Mention support for the 64-bit RISC-V instruction set (GOARCH=riscv64)
in the "Installing Go from source" document. Also sort the list of
supported instruction sets alphabetically.

Updates #27532

Change-Id: I07a443044a41a803853978dd7f7446de89ecceb5
Reviewed-on: https://go-review.googlesource.com/c/go/+/240377
Reviewed-by: Alberto Donizetti <[email protected]>

Author: tklauser

Diff for: doc/install-source.html

@@ -33,7 +33,7 @@ <h2 id="introduction">Introduction</h2>
 </p>
 
 <p>
-The Go compilers support twelve instruction sets:
+The Go compilers support the following instruction sets:
 
 <dl>
 <dt>
@@ -48,23 +48,29 @@ <h2 id="introduction">Introduction</h2>
 <dd>
   The <code>ARM</code> instruction set, 64-bit (<code>AArch64</code>) and 32-bit.
 </dd>
+<dt>
+  <code>mips64</code>, <code>mips64le</code>, <code>mips</code>,  <code>mipsle</code>
+</dt>
+<dd>
+  The <code>MIPS</code> instruction set, big- and little-endian, 64- and 32-bit.
+</dd>
 <dt>
   <code>ppc64</code>, <code>ppc64le</code>
 </dt>
 <dd>
   The 64-bit PowerPC instruction set, big- and little-endian.
 </dd>
 <dt>
-  <code>s390x</code>
+  <code>riscv64</code>
 </dt>
 <dd>
-  The IBM z/Architecture.
+  The 64-bit RISC-V instruction set.
 </dd>
 <dt>
-  <code>mips64</code>, <code>mips64le</code>, <code>mips</code>,  <code>mipsle</code>
+  <code>s390x</code>
 </dt>
 <dd>
-  The <code>MIPS</code> instruction set, big- and little-endian, 64- and 32-bit.
+  The IBM z/Architecture.
 </dd>
 <dt>
   <code>wasm</code>