1.0.24.32: undo parts of 1.0.24.26

1.0.24.32: undo parts of 1.0.24.26

No need for memory barriers when unlocking a spinlock on x86/x86-64.
The ordering rules and the cache coherency mechanism together
guarantee this. However, the compiler must be prevented from
reordering instructions with the unlock (at least in one direction).
This is now done in the runtime, but not in Lisp as the Lisp compiler
does no reordering.