doc/gccgo: update gcc status, remove double spaces

This change updates the status of gccgo 8 and gccgo 9, which are now
released.

It also replaces every instance of two-spaces with one space in text
paragraphs, which is the preferred style in Go documentation.

Fixes #34167

Change-Id: I94a4d85c06281f2623d39a68db7b6c95b5867999
Reviewed-on: https://go-review.googlesource.com/c/go/+/193842Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/gccgo_install.html

@@ -65,11 +65,15 @@ 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.
+The GCC 8 releases include a complete implementation of the Go 1.10.1
+release. The Go 1.10 runtime has now been fully merged into the GCC
+development sources, and concurrent garbage collection is fully
+supported.
+</p>
+
+<p>
+The GCC 9 releases include a complete implementation of the Go 1.12.2
+release.
 </p>
 
 <h2 id="Source_code">Source code</h2>