i#1551 Port DynamoRIO to ARM: rename segment_register to thread_register
- rename is_segment_register_initialized to is_thread_register_initialized
- rename read_selector to read_thread_register
- implement read_thread_register on ARM

R=bruening@google.com
BUG=1551

Review URL: https://codereview.appspot.com/176140043

git-svn-id: https://dynamorio.googlecode.com/svn/trunk@3018 49cc7528-f6fd-11dd-9d1a-b59b2e1864b6