[vm/ffi] Expand `Abi`s

This CL expands the ABIs in the FFI transform and VM from 3 groups to
18 individual ABIs in preparation of adding ABI-specific integer sizes
and structs.

This increases const lists with offsets in the kernel representation
from 3 to 18 elements. We do not expect significant size increases or
performance regressions from this. The constants are deduplicated in
Dart and the optimizer should optimize indexed lookups in const lists
away. However, landing this separately will enable us to verify that
assumption.

This also moves `Abi` to its own file and makes it an opaque class with
a predefined set of instances so that we do not depend on its internals.
That will enable us to keep `pkg/vm/lib/transformations/ffi/abi.dart`
consistent with the `Abi` abstraction to be introduced in `dart:ffi`
later for specifying ABI-specific integer sizes.

Bug: https://github.com/dart-lang/sdk/issues/42563
Bug: https://github.com/dart-lang/sdk/issues/42816

List of ABIs decided based on what's currently used (as Dart SDK target
platform, or Flutter target platform, or G3 target) and added
windows_arm64 in anticipation of
(https://github.com/flutter/flutter/issues/53120).
Excluded are macos_ia32 (https://github.com/dart-lang/sdk/issues/39810)
because we discontinued support; and windows_arm, fuchsia_arm,
fuchsia_ia32, ios_ia32, and macos_arm because these are
unlikely to be added.

TEST=pkg/front_end/testcases/*.expect
TEST=tests/ffi/*

Change-Id: I437707c18d8667490c063272a5f8a33d846e6061
Cq-Include-Trybots: luci.dart.try:vm-kernel-linux-debug-x64-try,vm-ffi-android-debug-arm-try,vm-kernel-mac-debug-x64-try,vm-kernel-nnbd-linux-debug-x64-try,vm-kernel-linux-debug-ia32-try,vm-kernel-precomp-linux-debug-x64-try,vm-kernel-reload-linux-debug-x64-try,vm-kernel-reload-rollback-linux-debug-x64-try,vm-kernel-win-debug-x64-try,vm-kernel-win-debug-ia32-try,vm-ffi-android-debug-arm64c-try,vm-kernel-mac-release-arm64-try,vm-precomp-ffi-qemu-linux-release-arm-try,vm-kernel-precomp-android-release-arm64c-try,vm-kernel-precomp-android-release-arm_x64-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/217184
Commit-Queue: Daco Harkes <dacoharkes@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Clement Skau <cskau@google.com>