Auto merge of #40777 - alexcrichton:update-mingw-compilers, r=aturon
appveyor: Upgrade MinGW toolchains we use
In debugging #40546 I was able to reproduce locally finally using
the literal toolchain that the bots were using. I reproduced the error maybe 4
in 10 builds. I also have the 6.3.0 toolchain installed through `pacman` which
has yet to have a failed build.
When attempting to reproduce the bug with the toolchain that this commit
switches to I was unable to reproduce anything after a few builds. I have no
idea what the original problem was, but I'm hoping that it was just some random
bug fixed somewhere along the way.
I don't currently know of a technical reason to stick to the 4.9.2 toolchains we
were previously using. Historcal 5.3.* toolchains would cause llvm to segfault
(maybe a miscompile?) but this seems to have been fixed recently. To me if it
passes CI then I think we're good.