thread_pthread.c: use get_stack
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44726 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
thread_pthread.c: use get_stack
place get_stack above others to get stack boundary information.
[ruby-core:60113] [Bug #9454]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44726 b2dd03c8-39d4-4d8f-98ff-823fe69b080e