s390x: Support tail calls

[uweigand]

This adds support for a newly defined tail-call ABI for s390x as well as supporting tail calls themselves.

We now use the tail-call ABI by default for Wasmtime, and enable tail calls by default.

This also allows getting rid of a number of special case and test exclusions for s390x.

Fixes: #6530

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[fitzgen]

Nice!

I didn't evaluate the details of the calling convention, as s390x isn't my area of expertise, but everything looks great in general, and I was very pleased to see the updates to the runtests and fuzzgen. Couple nitpick-y suggestions below, but I think this is basically good to go.

Thanks, Ulrich!!

[fitzgen]

What cases is this matching? I think it might be better to explicitly list them, rather than using _, so that if more MemArg variants are added in the future we don't forget to consider/update this case.

[fitzgen]

Can we add some documentation for what the nominal SP is and what its relation to the actual SP is? This was something that I always forgot the nitty-gritty details of in the old system, so if we are bringing part of it back, I think it would be helpful to have comments spelling those things out.

[fitzgen]

We don't always have a return area pointer, so I think this should return an Option<Reg> and let callers do the unwrap if they have knowledge of why it definitely exists in their specific case.

Suggested change

	/// Get the return area pointer register.
	pub fn ret_area_ptr(&self) -> Reg {
	self.ret_area_ptr.unwrap()
	}
	/// Get the return area pointer register, if any.
	pub fn ret_area_ptr(&self) -> Option<Reg> {
	self.ret_area_ptr
	}

[fitzgen]

And then this external constructor could be called unwrap_abi_ret_area_ptr or some such.

[fitzgen]

\o/

[uweigand]

Thanks for the review, @fitzgen ! The latest version should address all review comments.

[fitzgen]

Thanks!