i#1551 port to ARM: finish context switch gencode

Finishes off fcache_{enter,return}:
+ Implements insert_reachable_cti() for fcache_return
+ Updates append_save_clear_xflags to use r1 instead of r0

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

Review-URL: https://codereview.appspot.com/186580043