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>