Warn when defining a setf-function together with a setf-expander.

Patch by Douglas Katzman.
create_os_thread: put pthread stack inside alien-stack.

On !LISP_FEATURE_C_STACK_IS_CONTROL_STACK set pthread stack to
alien_stack, not control_stack.
Set up alien stack correctly on non-x86oids.

It's assumed that the C stack grows upward everywhere but X86oids,
which is not true. Define two new conditions,
ALIEN_STACK_GROWS_DOWNWARD and ALIEN_STACK_GROWS_UPWARD.

This fixes FFI issues on PPC.
Add a memory barrier inside pseudo-atomic on PPC.

Solves problems with allocation and multiple threads.