Added MSYS2 toolchains - gcc and clang - to appveyor CI builds

[luau-project]

What?

I have added new C toolchains -- gcc and clang -- provided by MSYS2 project to the Windows build on appveyor CI service.

Why?

On Windows, the toolchains provided by MSYS2 project are popular choices among programmers going for cross platform development. Therefore, since cminpack shares the same goal, it was a feature that I was looking to help with implementation.

How?

• Changed the build image on appveyor.yml to Visual Studio 2022 required by MSYS2 toolchains https://www.msys2.org/docs/ci#appveyor
• I reworked the appveyor.yml file to include x64 bit release builds for the following toolchains:
  • gcc x86_64;
  • gcc using the new universal C runtime for x64 bit (UCRT64);
  • clang for x64 bit (CLANG64).
• Added a preprocessor condition on test files && !defined(_UCRT) to fix the build for gcc using the new universal C runtime for x64 bit (UCRT64).

Testing?

The current appveyor.yml file is building just fine on every toolchain (MSVC, gcc x86_64, gcc UCRT64 and clang64).

Screenshots?

[devernay]

LGTM!