tests/fixedbugs: regression test for issue 100 (#129)

Adds a regression test based on klauspost/compress#186. This necessitated some related changes:

* Mark "RET" as a terminal instruction
* printer refactor to maintain compatibility with asmfmt
* Tweaks to other regression tests to ensure they are run correctly in CI

Updates #100 #65 #8

tests/fixedbugs/issue65/stub.go

@@ -0,0 +1,5 @@
+// Code generated by command: go run asm.go -out issue65.s -stubs stub.go. DO NOT EDIT.
+
+package issue65
+
+func Issue65()