Support IBM z/Architecture

[uweigand]

This adds support for the IBM z/Architecture (s390x-ibm-linux).

The status of the s390x backend in its current form is:

• Wasmtime is fully functional and passes all tests on s390x.
• All back-end features supported, with the exception of SIMD.
• There is still a lot of potential for performance improvements.
• Currently the only supported processor type is z15.

[cfallin]

Thanks for this @uweigand -- I'm very much looking forward to seeing this landed! I'll start reviewing soon; hopefully it won't take more than a week or two.

As a general note for others -- we have talked about the CI situation here, and for now there is still a bit of a gap in qemu's z15 support, so we won't yet have this tested on CI and will have to trust the filetests instead. This will hopefully be improved at some point, but it shouldn't stop us from considering this merge now, IMHO.

[cfallin]

This is fantastic work -- thank you @uweigand! I'm happy to finally see this patch after lots of anticipation over the past year :-)

I have a few comments below from a skim with more detailed reading in a few key spots -- the interesting issues are more likely to be in the glue and helpers than in the bulk of the lowering patterns, and the latter should be reasonably tested by the spec testsuite, which you have reported passes with this backend.

Others please weigh in here as well -- it's a lot of code to look over! -- but on first pass this should be an easy LGTM pending a few small comments below. Thanks again!

[uweigand]

Thanks for the review, @cfallin! I've checked in an update that should address all points you've raised.

[cfallin]

Thanks @uweigand -- everything appears to be resolved and I don't see any other major issues, so I think we can go ahead and merge! Of course we should improve the CI situation once we can; it would be nice to run the tests on every commit, once qemu is capable of doing that; but there's nothing particularly urgent about it. For now, ensuring that the backend builds and the filetests run should be enough to prevent major breakage.

[uweigand]

Thanks! I'll have a look at the current state of qemu. I'll also see if I can add support for z14, which would be useful anyway and might make it easier to use qemu.