Fix generic __memcpy_chk implementation.

- Clean up the labels (add .L to make them local).
- Change to using cfi directives.
- Fix unwinding of the __memcpy_chk fail path.

Bug: 18033671
Change-Id: I12845f10c7ce5e6699c15c558bda64c83f6a392a
Merge "Fix generic __memcpy_chk implementation."