This also fixes a bug in 'InitializeCallBuffer', where it wouldn't claim enough
slots for each parameter. This caused the Simd128 instruction selector test to
only claim 3 slots (rather than 4) and then perform an unnecessary padding poke.

v8_Default_embedded_blob_size from the generated file gen/embedded.S
  Before: 4957056
   After: 4954368

This gives a 0.05% size decrease.

Change-Id: Ic9bb998fb8a9111fb90e1c3e537ea0f2a5fa7b33
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1617665Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Martyn Capewell <martyn.capewell@arm.com>
Cr-Commit-Position: refs/heads/master@{#61649}