[xla:cpu] Add support for emitting constant allocations with thunk-based runtime
[xla:cpu] Add support for emitting constant allocations with thunk-based runtime

To use thunk runtime in tests:

bazel test //xla/tests:copy_test_cpu --test_env=XLA_FLAGS="--xla_dump_to=/tmp/xla-cpu --xla_cpu_use_thunk_runtime=true" --test_env=TF_CPP_VMODULE=copy_thunk=3 --test_output=all
