merge revision(s) 86022167,47a1872c: [Backport #16681]

	Switch to download libffi source package to github releases from
	 sourceware.org


	Use osuosl instead of GitHub releases

	  Because the package provided by GitHub releases is different from sourceware.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67842 b2dd03c8-39d4-4d8f-98ff-823fe69b080e