Add support for basic NEPTUNE env with nvhpc@26.5

[climbfuji]

Description

This PR adds support for building a basic NEPTUNE environment with nvhpc@26.5, similar to the llvm@22 NEPTUNE environment. A large number of packages need to be built with GCC instead of NVHPC.

Because NVIDIA is updating their compiler suite from the legacy PGI backend to the modern LLVM IR in the near future, it doesn't make sense to try to fix the many build errors with NVHPC. Also, NVIDIA's support to fix bugs in the current compilers is limited - they defer users to the upcoming LLVM-based compilers.

Dependencies

• JCSDA-fork only: Add support for ip@5.4.0 with nvhpc@2.6.5 spack-packages#79

Issues addressed

NRL-internal issue

Applications affected

None (this is an experimental setup)

Systems affected

None

Testing

• CI: Note whether the automatic tests (GitHub actions tests that run automatically for every commit) pass or not
  • GitHub actions CI tests pass (CI tests not affected by this change)
  • GitHub actions CI tests do not pass (provide explanation)
  • GitHub actions CI tests skipped (provide explanation if necessary)
• New tests added: List and describe any new tests added to GitHub actions
  • ...
• Additional testing: Add information on any additional tests conducted
  • ...

Checklist

• This PR addresses one issue/problem/enhancement or has a very good reason for not doing so.
• These changes have been tested on the affected systems and applications. work in progress
• All dependency PRs/issues have been resolved and this PR can be merged.
• All necessary updates to the documentation (spack-stack wiki) will be made when this PR is merged