[vm] Handle entry point object pool entries only in AOT
[vm] Handle entry point object pool entries only in AOT

Avoid touching stubs in JIT mode when reading object pool
as stubs may not be initialized yet. This may happen if
using a core-jit snapshot.

TEST=ci

Change-Id: I598ef2ca4b6b22fb1bb560ad25b6ada9939799a0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/216924
Reviewed-by: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@google.com>
Commit-Queue: Alexander Markov <239e774fd29abc1267878f57afe1a8a03e0ca887@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk