Drop MinGW support of MJIT
Stop supporting MJIT on MinGW from Ruby 3.2
Compilers on MinGW behave weirdly and MinGW-specific C code is sometimes required as a workaround. Despite the high maintenance cost, nobody seems to be using MJIT on MinGW.
- I installed Ruby 3.1 with RubyInstaller2, which I believe is the most popular method to install MinGW Ruby, and MJIT didn't work in that environment.
- MinGW is known to be very slow at compiling C code. It's improbable that the architecture of MJIT performs well on that platform.
- In other platforms, compiling an MJIT-generated .c file to a .so file works fine, but it fails on MinGW without any error message. So we have to compile .c to .o first and then build a .so file, just for MinGW.