thread_pthread.c: get current main thread stack size

* thread_pthread.c: get current main thread stack size, which may
  be expanded than allocated size at initialization, by rlimit().
  [ruby-core:60113] [Bug #9454]

git-svn-id: svn+ssh://svn.ruby-lang.org/ruby/trunk@44712 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-01-27

git-svn-id: svn+ssh://svn.ruby-lang.org/ruby/trunk@44713 b2dd03c8-39d4-4d8f-98ff-823fe69b080e