[vm/ffi] Fix MacOS and iOS arm64 after struct stack alignment

On MacOS and iOS arm64, non-struct stack arguments are aligned to their
own size rather than word size. However, after a struct, the first non-
struct argument is word-aligned. This is achieved by doing alignment of
the stack also _after_ every argument passed on the stack.

Doing alignment _after_ every argument passed on the stack is a no-op
in all other calling conventions because there all stack arguments are
word-size aligned.

TEST=runtime/vm/compiler/ffi/unit_tests/structPacked/arm64_macos.expect
TEST=tests/ffi_2/function_structs_by_value_generated_test.dart
TEST=tests/ffi_2/function_callbacks_structs_by_value_generated_test.dart

Change-Id: Iffa6625cd40774fc1d2ca9180a42be7200d93a01
Cq-Include-Trybots: luci.dart.try:vm-kernel-mac-release-arm64-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/203303
Reviewed-by: Tess Strickland <sstrickl@google.com>
Commit-Queue: Daco Harkes <dacoharkes@google.com>