i#1551 port to ARM: top-level generated code

Fills in some top-level gencode pieces:
+ Due to the impracticality of absoulte addresses on ARM, we follow
  64-bit's lead and use shared gencode for thread-private code.  We update
  USE_SHARED_GENCODE_ALWAYS() and arch_thread_init() and arch_thread_exit()
  to reflect this (avoiding creating thread-private gencode).
+ Implements SET_TO_NOPS for filling in between gencode routines

Xref original CL: https://codereview.appspot.com/188460043/

Review-URL: https://codereview.appspot.com/192890043