This patch updates the instruction selector and code generator to pad arguments
for arm64 and drop an even number of slots when dropping the arguments. It also
updates the builtins that handle arguments. These changes need to be made at
the same time.

It also adds some tests for forwarding varargs, as this was affected by the
builtin changes and the existing tests did not catch all issues.

Bug: v8:6644
Change-Id: I81318d1d1c9ab2568f84f2bb868d2a2d4cb56053
Reviewed-on: https://chromium-review.googlesource.com/829933
Commit-Queue: Georgia Kouveli <georgia.kouveli@arm.com>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50259}