web/downloads: improve the explanations for the various toolchain features.

- spend some more time explaining "host" and "target"
- improve wording for C++ exception models
- threading model is for C11, not C++11
- explain that the GCC threading model does nothing for apps not using C11

We finally got some constructive feedback for this section of the website!

git-svn-id: https://svn.code.sf.net/p/mingw-w64/code@6335 4407c894-4637-0410-b4f5-ada5f102cad1
web/download: revert promoting mingw-builds.

This reverts two commits:
- r6292: web/download: move mingw-builds as the first toolchain.
- r6294: web/download: note that for windows hosts, mingw-builds should
         be preferred

I've decided to mostly stop touching the download page.
I had done these two changes after having asked Kaï about it and with
the idea that there would be more changes to the corresponding section
on the download page.

I have no idea why (and probably don't want to know) but it's clear that
this section of the page doesn't change without me. And with much more
work and frustration than it should.

I do not feel comfortable making more visible something that will not be
updated.

Revert this commit if the situation changes or you believe I'm under the
wrong impression.

git-svn-id: https://svn.code.sf.net/p/mingw-w64/code@6336 4407c894-4637-0410-b4f5-ada5f102cad1
downloads: "yypkg mingw-builds" becomes "win-builds".

To avoid the name conflict with "mingw-builds", I'm renaming my project to
"win-builds".

Mention the support for GCC 4.8.1 and CRT v3 which will be available as
an alpha by tomorrow.

Expand the description too.

git-svn-id: https://svn.code.sf.net/p/mingw-w64/code@6337 4407c894-4637-0410-b4f5-ada5f102cad1