This adds infrastructure to allow implementing call and return
instructions in ISLE, and migrates the s390x back-end.
To implement ABI details, this patch creates public accessors
for `ABISig` and makes them accessible in ISLE. All actual
code generation is then done in ISLE rules, following the
information provided by that signature.
[ Note that the s390x back end never requires multiple slots for
a single argument - the infrastructure to handle this should
already be present, however. ]
To implement loops in ISLE rules, this patch uses regular tail
recursion, employing a `Range` data structure holding a range
of integers to be looped over.
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