Test stackmap support for floating point types.

It appears that float support is complete, or at least, the stackmap records
emitted are not inconceivable (I must admit that I don't know about many of the
architectures under test here).

One curiosity, the SystemZ tests highlight an undocumented (or maybe incorrect)
quirk of the stackmap format: in the case of a Register record, the Offset or
SmallConstant field can encode a sub-register index! I've only ever seen this
field zero for Register entries up until now.