"1.0.41.2": threads: Add memory-barrier framework.

"1.0.41.2": threads: Add memory-barrier framework.

  * New file, src/code/barrier.lisp, containing the baseline
implementation.

  * 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.