Align arm64 stacks to 16 bytes in __bionic_clone.

Also ensure that arm64/x86-64/x86 assembler uses local labels.
(There are are so many non-local labels in arm that fixing them
seems out of scope.)

Also synchronize the __bionic_clone.S comments.

Change-Id: I03b4f84780d996b54d6637a074638196bbb01cd4
5 files changed