x86-64 needs these CFI directives to stop unwinding here.
I've also cleaned up the assembler a little, and made x86 and x86-64
a little more alike.
Bug: 15195760
(cherry picked from commit aeb3016f8132689d1b49d30056005b667e3d2d0e)
Change-Id: I511fe238b1ef1a1c16aef735f93bbee9accb0689