The original CL introduced a test which uses a random number generator.
I disable the test for now, which is okay because this CL adds to a
work-in-progress feature anyways, and I will fix the problem in another
CL.

Original description:
Add the ability to return (multiple) return values on the stack:

- Extend stack frames with a new buffer region for return slots.
  This region is located at the end of a caller's frame such that
  its slots can be indexed as caller frame slots in a callee
  (located beyond its parameters) and assigned return values.
- Adjust stack frame constructon and deconstruction accordingly.
- Extend linkage computation to support register plus stack returns.
- Reserve return slots in caller frame when respective calls occur.
- Introduce and generate architecture instructions ('peek') for
  reading back results from return slots in the caller.
- Aggressive tests.
- Some minor clean-up.

So far, only ia32 and x64 are implemented.

Change-Id: I8b03fc4e53946daaa0e14a34603f4824a04fad7e
Reviewed-on: https://chromium-review.googlesource.com/819557Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50031}