[wasm] Make multi-return tests wasm-specific

The multi-return tests and fuzzer used a custom call descriptor which
was based on the default RegisterConfiguration. This meant that for the
tests, all available registers could be used to pass parameters and to
return values. This caused a problem, because in some cases we need a
scratch register in the frame deconstruction.

With this CL I change both the tests and the fuzzer to use the
WebAssembly call descriptor. Thereby we only use 2 registers for
returns, and one of the other registers can be used as scratch
register.

WebAssembly is the only use case at the moment which wants to return
values not only through registers but also over the stack. Therefore
I think it's acceptable to only test the WebAssembly usecase.

R=mstarzinger@chromium.org

Bug: chromium:813288
Change-Id: I31bed757af5f3e8589d2b3dfb6f0112ddecd1a20
Reviewed-on: https://chromium-review.googlesource.com/970656Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52099}