Spack changes for laghos and its dependencies

[rfhaque]

This PR adds changes to the spack packages for laghos and its dependencies in preparation to upstream to the spack-packages repository. The PR

• Adds support for c++20 to the cce compiler (required for RAJA)
• Adds support for the develop versions of RAJA, Umpire and camp
• Adds a variant to use the default GPU stream in camp. Depends on Add cmake option for CAMP_USE_PLATFORM_DEFAULT_STREAM camp#205

Adding/modifying a benchmark (docs: Adding a Benchmark)

• If package.py upstreamed to Spack is insufficient, add/modify repo/benchmark_name/package.py plus: create, self-assign, and link here a follow up issue with a link to the PR in the Spack repo.