spsc-queue: add spsc-relacy build

spsc-queue: add spsc-relacy build

Now we can test a version of spsc-queue using Relacy. There were some
small modifications necessary, so I duplicated the source code, renamed
with "-relacy" in the name.

To use it, you need to do something like the following, from the
spsc-queue directory:

  make RELACYPATH=/path-to-relacy
  ./spsc-relacy