doc: update binary requirements

FreeBSD 9.3.
Add Linux arm64. (required second line)
Clarify glibc requirement now that we have second line in notes.
OS X to macOS

Updates #20850

Change-Id: I684d464ed32a072081726b7c805a346c22c42f97
Reviewed-on: https://go-review.googlesource.com/47252Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/install.html

@@ -33,7 +33,7 @@ system and architecture, try
 <h2 id="requirements">System requirements</h2>
 
 <p>
-Go binary distributions are available for these supported operating systems and architectures.
+Go <a href="https://golang.org/dl/">binary distributions</a> are available for these supported operating systems and architectures.
 Please ensure your system meets these requirements before proceeding.
 If your OS or architecture is not on the list, you may be able to
 <a href="/doc/install/source">install from source</a> or
@@ -47,10 +47,10 @@ If your OS or architecture is not on the list, you may be able to
 <th align="center">Notes</th>
 </tr>
 <tr><td colspan="3"><hr></td></tr>
-<tr><td>FreeBSD 8-STABLE or later</td> <td>amd64, 386</td> <td>Debian GNU/kFreeBSD not supported</td></tr>
-<tr><td>Linux 2.6.23 or later with glibc</td> <td>amd64, 386, arm, s390x, ppc64le</td> <td>CentOS/RHEL 5.x not supported</td></tr>
-<tr><td>Mac OS X 10.8 or later</td> <td>amd64</td> <td>use the clang or gcc<sup>&#8224;</sup> that comes with Xcode<sup>&#8225;</sup> for <code>cgo</code> support</td></tr>
-<tr><td>Windows XP or later</td> <td>amd64, 386</td> <td>use MinGW gcc<sup>&#8224;</sup>. No need for cygwin or msys.</td></tr>
+<tr><td>FreeBSD 9.3 or later</td> <td>amd64, 386</td> <td>Debian GNU/kFreeBSD not supported</td></tr>
+<tr valign='top'><td>Linux 2.6.23 or later with glibc</td> <td>amd64, 386, arm, arm64,<br>s390x, ppc64le</td> <td>CentOS/RHEL 5.x not supported.<br>Install from source for other libc.</td></tr>
+<tr><td>macOS 10.8 or later</td> <td>amd64</td> <td>use the clang or gcc<sup>&#8224;</sup> that comes with Xcode<sup>&#8225;</sup> for <code>cgo</code> support</td></tr>
+<tr><td>Windows XP SP2 or later</td> <td>amd64, 386</td> <td>use MinGW gcc<sup>&#8224;</sup>. No need for cygwin or msys.</td></tr>
 </table>
 
 <p>