Release SYCL back-end features

[rhornung67]

Due to interest expressed by ECP application users in trying the RAJA SYCL back-end, we deploy features in development in RAJA releases as they become ready. For the initial release (v0.14.0 -- we are shooting for the end of April 2021), we will deploy the following:

• [x] forall (probably non-resource API only at first)
• [x] reductions
• [x] basic kernel functionality

A PR has been started: https://github.com/LLNL/RAJA/pull/814

In addition, we will do the following:

• [x] make a camp PR to add a SYCL resource
• [ ] provide basic usage documentation in the RAJA User Guide (code compilation instructions as needed, SYCL execution policies, code examples, etc. Initial release will contain execution policies for trivially-copyable and non-trivially copyable object support.
• [x] testing of released features on Travis using the open source Intel OneAPI compiler image
• [ ] testing of released features on Gitlab CI (targeting AMD GPUs)

Later releases will include:

• [ ] atomic operations
• [x] support for additional kernel capabilities (tiling, etc.)
• [x] launch/teams support
• [ ] scans
• [ ] workgroup/kernel fusion
• [ ] graph API (in development)

[homerdin]

I've opened a new PR #1034 with the SYCL backend code. This includes the features that were discussed.

The camp PR for the SYCL resource is at https://github.com/LLNL/camp/pull/48.