merge revision(s) 44670,44671,44673,44675: [Backport #8783]

	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.
	* 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]

	* thread_pthread.c (rb_thread_create_timer_thread): fix for platforms
	  where PTHREAD_STACK_MIN is a dynamic value and not a compile-time
	  constant.  [ruby-dev:47911] [Bug #9436]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_1@45119 b2dd03c8-39d4-4d8f-98ff-823fe69b080e