add roundtrip disasm tests for all cpus

test/dasm/readme.txt

@@ -0,0 +1,9 @@
+
+Per CPU, a test binary is produced (using the assembler), which should contain
+all possible instructions. That file is then disassembled, and assembled again,
+and finally the resulting binary compared with the binary produced in the first
+step.
+
+Given that we assume the assembler works (this is tested in other/previous
+tests), this proves that the disassembler works, and can produce output that the
+assembler will understand - and produce an identical binary from.