doc: adjust binary install page supported system list

Make clear that this list is the list of supported systems
for binary distributions, and that other systems may be
able to build the distribution from source, in addition
to using gccgo.

Drop freebsd/arm from the list on this page.
We have never issued a binary distribution for freebsd/arm,
and we're not going to start in Go 1.5, since we don't even
have a working builder for it.

Drop freebsd/386 from the list on the page,
because we are unable to build binary distributions, per adg.

I think the wording here should probably be revised further,
but not now.

Change-Id: Ib43b6b64f5c438bfb9aa4d3daa43393f1e33b71f
Reviewed-on: https://go-review.googlesource.com/13690Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

doc/install.html

@@ -33,11 +33,11 @@ system and architecture, try
 <h2 id="requirements">System requirements</h2>
 
 <p>
-The <code>gc</code> compiler supports the following operating systems and
-architectures. Please ensure your system meets these requirements before
-proceeding. If your OS or architecture is not on the list, it's possible that
-<code>gccgo</code> might support your setup; see
-<a href="/doc/install/gccgo">Setting up and using gccgo</a> for details.
+Go binary distributions 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
+<a href="/doc/install/gccgo">use gccgo instead</a>.
 </p>
 
 <table class="codetable" frame="border" summary="requirements">
@@ -47,9 +47,9 @@ proceeding. If your OS or architecture is not on the list, it's possible that
 <th align="center">Notes</th>
 </tr>
 <tr><td colspan="3"><hr></td></tr>
-<tr><td>FreeBSD 8-STABLE or later</td> <td>amd64, 386, arm</td> <td>Debian GNU/kFreeBSD not supported; FreeBSD/ARM needs FreeBSD 10 or later</td></tr>
-<tr><td>Linux 2.6.23 or later with glibc</td> <td>amd64, 386, arm</td> <td>CentOS/RHEL 5.x not supported; no binary distribution for ARM yet</td></tr>
-<tr><td>Mac OS X 10.7 or later</td> <td>amd64</td> <td>use the gcc<sup>&#8224;</sup> that comes with Xcode<sup>&#8225;</sup></td></tr>
+<tr><td>FreeBSD 8-STABLE or later</td> <td>amd64</td> <td>Debian GNU/kFreeBSD not supported</td></tr>
+<tr><td>Linux 2.6.23 or later with glibc</td> <td>amd64, 386, arm</td> <td>CentOS/RHEL 5.x not supported; install from source for ARM</td></tr>
+<tr><td>Mac OS X 10.7 or later</td> <td>amd64</td> <td>use the clang or gcc<sup>&#8224;</sup> that comes with Xcode<sup>&#8225;</sup></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>
 </table>