"18.104.22.168": threads: Add memory-barrier framework.
* New file, src/code/barrier.lisp, containing the baseline
* Added the barrier functions to the compiler function database.
* Export the interface macro, BARRIER, from SB!THREAD and the
underlying barrier functions from SB!VM.
* Document a new architecture-dependent build-time feature,
MEMORY-BARRIER-VOPS, for controlling the behavior and inlining of
the interpreter stubs for the barrier functions.