doc: update GCC release info in gccgo_install

Fixes #22077

Change-Id: I8d1b18e78719514fa2a410e85c9d3934c47cd2fe
Reviewed-on: https://go-review.googlesource.com/66930Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

doc/gccgo_install.html

@@ -59,10 +59,17 @@ should not be visible to Go programs.
 </p>
 
 <p>
-The GCC 7 releases are expected to include a complete implementation
-of the Go 1.8 user libraries.  As with earlier releases, the Go 1.8
-runtime is not fully merged, but that should not be visible to Go
-programs.
+The GCC 7 releases include a complete implementation of the Go 1.8.1
+user libraries.  As with earlier releases, the Go 1.8 runtime is not
+fully merged, but that should not be visible to Go programs.
+</p>
+
+<p>
+The GCC 8 releases are expected to include a complete implementation
+of the Go 1.10 release, depending on release timing.  The Go 1.10
+runtime has now been fully merged into the GCC development sources,
+and concurrent garbage collection is expected to be fully supported in
+GCC 8.
 </p>
 
 <h2 id="Source_code">Source code</h2>