because it had already been SEGV-ed in pull request'S CI
and has never worked on trunk either.
Please make sure it does not SEGVs on your MinGW environment or pull
request before enabling native fiber on MinGW.
appveyor.yml: reverted commits to make CI green with native fiber
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65892 b2dd03c8-39d4-4d8f-98ff-823fe69b080e