pb foreign-function interface: refine union handling for s390x
#990
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With pb, foreign calls in general use libffi, which does not provide support for `union` arguments and results. The binding in "c/ffi.c" approximates `union` handling by constructing a `struct` description that is likely to be treated the same in the platform's ABI. The approximation was wrong for s390x in the case of `union`s that contain only floating-point fields, so add a special case for s390x. There are surely other platforms that would need similar treatment, but s390x is relevant because there's no native-code backend for that architecture.
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With pb, foreign calls in general use libffi, which does not provide support for
unionarguments and results. The binding in "c/ffi.c" approximatesunionhandling by constructing astructdescription that is likely to be treated the same in the platform's ABI. The approximation was wrong for s390x in the case ofunions that contain only floating-point fields, so add a special case for s390x. There are surely other platforms that would need similar treatment, but s390x is relevant because there's no native-code backend for that architecture.Related to #956