S390 [wasm-simd]: Fix Float32 <-> Int32 conversions on older machines

vcgd/vcdg with 32-bit FP inputs are only supported
on z15 and above. For older machines we need to use scalar
instructions.

This is a partial revert of this CL: https://crrev.com/c/2697389

Change-Id: I61deb9357efd424c3b94dddc8be37e7e4c42d334
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2936640


Reviewed-by: Junliang Yan <junyan@redhat.com>
Commit-Queue: Milad Fa <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/master@{#74939}