gc.c: GC.latest_gc_info reports most interesting major_by flag

	* gc.c (gc_info_decode): Use :major_by=>:nofree as fallback reason
	  when other trigger conditions are present.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44171 b2dd03c8-39d4-4d8f-98ff-823fe69b080e