Remove scavenging of read-only space.
`*SCAVENGE-READ-ONLY-SPACE*` is always set to `NIL`, and it's highly unlikely any one ever sets it. I (rtoy) haven't done that in decades at least. So, remove this static symbol, and remove the C code that tests for this to determine if the read-only space should be scavenged. This requires a very simple cross-compile to remove the symbol. See boot-2020-04-1.lisp for very simple instructions. We currently on do this for x86 since we can't test on sparc right now. Update CI to do a cross-compile. Addresses #89