thread_pthread.c: timer thread stack size

* thread_pthread.c (rb_thread_create_timer_thread): define the stack
  size for timer thread at compile time.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44670 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
thread_pthread.c: expand timer thread stack size

* thread_pthread.c (rb_thread_create_timer_thread): expand timer
  thread stack size to get rid of segfault on FreeBSD/powerpc64.
  based on the patch by Steve Wills at [ruby-core:59923].
  [ruby-core:56590] [Bug #8783]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44671 b2dd03c8-39d4-4d8f-98ff-823fe69b080e