[vm] Use `SnapshotBehavior` for switchable call pool entries

Follow up of:
* https://dart-review.googlesource.com/c/sdk/+/328861
* https://dart-review.googlesource.com/c/sdk/+/331087
* https://dart-review.googlesource.com/c/sdk/+/331120

The pool entries for switchable calls have their pool entries
transformed during snapshot loading in AOT. This behavior is now folded
into `SnapshotBehavior`.

Because snapshot behavior is set on the call sites, and the call sites
are already AOT specific, no conditionals inside app_snapshot.cc are
needed anymore.

The Arm64 implementation does not use the assembler for implicitly
adding pool entries, it does so explictly and loads two pool entries
in one load.

We could consider doing these explicit loads in the other architectures
as well. This would avoid adding the snapshot_behavior argument to
`LoadUnqiueObject` for the other architectures.

IA32 has no AOT support.

TEST=precomp bots

Change-Id: I75f70e00ca44f6545694d9b59e3ad08ad8bb3f8e
Cq-Include-Trybots: luci.dart.try:vm-aot-android-release-arm64c-try,vm-aot-android-release-arm_x64-try,vm-aot-linux-debug-simarm_x64-try,vm-aot-linux-debug-simriscv64-try,vm-aot-linux-debug-x64-try,vm-aot-linux-release-arm64-try,vm-aot-linux-release-simarm_x64-try,vm-ffi-qemu-linux-release-riscv64-try,vm-linux-debug-simriscv64-try,vm-aot-mac-product-arm64-try,vm-aot-win-debug-arm64-try,vm-ffi-android-debug-arm64c-try,vm-ffi-android-debug-arm-try,vm-ffi-qemu-linux-release-arm-try,vm-aot-win-debug-x64c-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/331101
Commit-Queue: Daco Harkes <dacoharkes@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>