doc: mention s390x in environment section of install-source.html

Change-Id: I521ba94356165efc3252ca4b3d364a2bbabd4aab
Reviewed-on: https://go-review.googlesource.com/58010
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/install-source.html

@@ -471,8 +471,9 @@ Choices for <code>$GOARCH</code> are
 <code>amd64</code> (64-bit x86, the most mature port),
 <code>386</code> (32-bit x86), <code>arm</code> (32-bit ARM), <code>arm64</code> (64-bit ARM),
 <code>ppc64le</code> (PowerPC 64-bit, little-endian), <code>ppc64</code> (PowerPC 64-bit, big-endian),
-<code>mips64le</code> (MIPS 64-bit, little-endian), and <code>mips64</code> (MIPS 64-bit, big-endian).
-<code>mipsle</code> (MIPS 32-bit, little-endian), and <code>mips</code> (MIPS 32-bit, big-endian).
+<code>mips64le</code> (MIPS 64-bit, little-endian), <code>mips64</code> (MIPS 64-bit, big-endian),
+<code>mipsle</code> (MIPS 32-bit, little-endian), <code>mips</code> (MIPS 32-bit, big-endian), and
+<code>s390x</code> (IBM System z 64-bit, big-endian).
 The valid combinations of <code>$GOOS</code> and <code>$GOARCH</code> are:
 <table cellpadding="0">
 <tr>
@@ -536,6 +537,9 @@ The valid combinations of <code>$GOOS</code> and <code>$GOARCH</code> are:
 <td></td><td><code>linux</code></td> <td><code>mips64le</code></td>
 </tr>
 <tr>
+<td></td><td><code>linux</code></td> <td><code>s390x</code></td>
+</tr>
+<tr>
 <td></td><td><code>netbsd</code></td> <td><code>386</code></td>
 </tr>
 <tr>