i#1551 Port DynamoRIO to ARM: add steal reg API

On ARM, we need steal a register for DynamoRIO usage, e.g., TLS access.
This is the API we plan to support, including
- runtime option: -steal_reg
- API: dr_get_stolen_reg, dr_insert_get_stolen_reg, and reg_is_stolen

Review URL: https://codereview.appspot.com/178700043