make NVHPC a full toolchain with nvidia-compilers, NVHPCX and NVBLAS

[lexming]

Fixes #4853

Enhance NVHPC to be a full toolchain:

• compilers provided by new nvidia-compilers subtoolchain
  • equivalent to current NVHPC
  • nvidia-compilers will have their own easyconfigs
• mpi provided by new NVHPCX subtoolchain
  • NVHPCX is bundled in NVHPC so it will not exist with their own easyconfigs
• linalg provided by new NVBLAS subtoolchain
  • NVBLAS is bundled in NVHPC so it will not exist with their own easyconfigs

I also added some extra features related to NVHPC:

• cuda_compute_capabilities can be defined from environment variable $EBNVHPCCUDACC: this environment variable can be set by nvidia-compilers or NVHPC if they are installed with a given cuda_compute_capabilities, avoiding the need to manually set cuda_compute_capabilities for every installation in that toolchain.
• CUDA templates (e.g. %(cudaver)s) will be defined from environment variable $EBNVHPCCUDAVER: this environment variable will be set by nvidia-compilers or NVHPC, allowing to use those templates in all easyconfigs in the toolchain without having to depend on CUDA.
• new template %(cuda_cc_nvhpc)s for CUDA compute capabilities with the format expected by NVHPC

[lexming]

Test Reports

• Tests with existing easyconfigs:
  refactor NVHPC easyblock into NvidiaBase, nvidia-compilers and NVHPC easybuild-easyblocks#3788 (comment)
• Tests with new easyconfigs:
  split NVHPC v25 easyconfigs into nvidia-compilers easybuild-easyconfigs#23125 (comment)