thread_pthread.c: dup code

* thread_pthread.c (get_stack): merge duplicated code split by ifdef.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42715 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
thread_pthread.c: get_stack on HP-UX

* thread_pthread.c (hpux_attr_getstackaddr): basic support for the
  get_stack() under HP-UX.  based on the patch by michal@rokos.cz
  (Michal Rokos) at [ruby-core:56645].  [Feature #8793]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42716 b2dd03c8-39d4-4d8f-98ff-823fe69b080e