Add backend-neutral aliases for non-sized direct_unchecked device policies

[artv3]

RAJA provides backend-neutral device_* aliases for many CUDA/HIP execution and
kernel loop policies. While updating I found that the
non-sized direct_unchecked block/thread policy aliases do not appear to have
exact backend-neutral equivalents.

Current backend-specific aliases:

RAJA::cuda_block_x_direct_unchecked
RAJA::cuda_block_y_direct_unchecked
RAJA::cuda_block_z_direct_unchecked
RAJA::cuda_thread_x_direct_unchecked
RAJA::cuda_thread_y_direct_unchecked
RAJA::cuda_thread_z_direct_unchecked

RAJA::hip_block_x_direct_unchecked
RAJA::hip_block_y_direct_unchecked
RAJA::hip_block_z_direct_unchecked
RAJA::hip_thread_x_direct_unchecked
RAJA::hip_thread_y_direct_unchecked
RAJA::hip_thread_z_direct_unchecked

Requested backend-neutral equivalents:

RAJA::device_block_x_direct_unchecked
RAJA::device_block_y_direct_unchecked
RAJA::device_block_z_direct_unchecked
RAJA::device_thread_x_direct_unchecked
RAJA::device_thread_y_direct_unchecked
RAJA::device_thread_z_direct_unchecked

RAJA currently provides sized aliases:

RAJA::device_block_size_x_direct_unchecked<N>
RAJA::device_thread_size_x_direct_unchecked<N>

Those are useful, but they are not exact replacements for the non-sized forms.