Having the version in `get_run_handlers` meant that a RUN line without the
version in it would not match, and therefore UTC would generate a useless
catch-all check of the form:
```
;; NOTE: These prefixes are unused and the list is autogenerated. Do not add tests below this line:
; CHECK: {{.*}}
```
This patch also adds `arm64_32` as a base-level run handler, and assumes it
will always apply to a darwin target (which is currently the case for all
arm64_32-* triples I see in tree currently).
4ee950e75d