[c++2b] Add option -std=c++2b to enable support for potential C++2b features.

[rsmith]

There are some mechanical changes we should make to our documentation too, particularly www/cxx_status.html.

[curdeius]

In D92547#2430428, @rsmith wrote:

There are some mechanical changes we should make to our documentation too, particularly www/cxx_status.html.

Ok, I will soon send a patch to update cxx_status adding C++2b.