Remove mingw toolset

[Jarod42]

What does this PR do?

It removes toolset mingw
which is mostly an alias of "gcc on windows" (for filtering).

How does this PR change Premake's behavior?

filter { "toolset:mingw" } is broken, and alternative should be used, as filter { "system:windows", "toolset:not msc" }

Anything else we should know?

• Detecting correct mingw-w64/llvm-mingw/msys2 is currently not done anyway
• Some mingw uses clang and not gcc

So some works should be done if we want to handle mingw correctly.

Did you check all the boxes?

• Focus on a single fix or feature; remove any unrelated formatting or code changes
• Add unit tests showing fix or feature works; all tests pass
• Mention any related issues (put closes #XXXX in comment to auto-close issue when PR is merged)
• Follow our coding conventions
• Minimize the number of commits
• Align documentation to your changes

[nickclark2016]

What about LLVM with Visual Studio? How should this be handled?

[Jarod42]

So big build matrix! It seems I would have to add extra CI config.

I honestly don't know

• if clang under visual should have the link
• if clang under mingw should have the link.
  It seems to be usage from curl.

I can use filter { "system:windows", "toolset:gcc" } to not change those configs

[redorav]

This feels like a bigger change like we discussed, does this solve an immediate problem?

[Jarod42]

Discussion was adding extra API which would be a big change IMO.
Here I (propose to) remove the strange toolset mingw (which is no even web documented)

[Jarod42]

Added #2576 for LLVM with visual.
Once merged, I would rebase this PR which should answer the original question.

[nickclark2016]

Requesting a follow-up PR to allow for deprecation of "allowed" values in "newoption" (via an API)