Fix unwinding through x86-64 __bionic_clone.

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
Change-Id: I40f92c007843c29c933bb6876fe2b4611e1b946b
2 files changed