Ruby

fiber: fix machine stack marking when FIBER_USE_NATIVE is 0

• cont.c (cont_mark): mark Fiber machine stack correctly when
  FIBER_USE_NATIVE is 0
• test/ruby/test_fiber.rb (test_mark_fiber): new test
  [Bug #13875] [ruby-core:82681]

This bug appears to be introduced with r59557.
("refactoring Fiber status")

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59785 b2dd03c8-39d4-4d8f-98ff-823fe69b080e