In the default case, we don't need to use the stack, we can save r7 with
ip register (that what does eglibc).
This allow to fix vfork data corruption
(see 3884bfe9661955543ce203c60f9225bbdf33f6bb), because vfork now don't
use the stack.
The kernel has supported this syscall for quite some time now,
but bionic did not. Now that there is a need for it, let's
add it to bionic.
Change-Id: Ifcef3e46f1438d79435b600c4e6063857ab16903