Axom: Update ROCm root var

[chapman39]

Similar to #2365, this PR updates Axom var rocm_root to move a previous directory back from llvm-amdgpu prefix. The assumption is the prefix is /opt/rocm-6.4.2/llvm.

If other's set the llvm-amdgpu prefix differently than below, maybe we can handle both cases.

    llvm-amdgpu:
      externals:
      - spec: [email protected]
        prefix: /opt/rocm-6.4.2/llvm
        extra_attributes:
          compilers:
            c: /opt/rocm-6.4.2/llvm/bin/amdclang
            cxx: /opt/rocm-6.4.2/llvm/bin/amdclang++
            fortran: /opt/rocm-6.4.2/llvm/bin/amdflang

Also adds unrelated fix to an incorrect version in MFEM from a previous PR of mine #2363 (comment)

[chapman39]

@adrienbernede First of all, thank you for updating the Axom Spack package here. I'm curious if you need to set llvm-amdgpu prefix like /opt/rocm-6.4.2 for your use-case, or if /opt/rocm-6.4.2/llvm works for you.

I haven't tried in awhile but I think other packages failed when I set the llvm-amdgpu prefix to just /opt/rocm-6.4.2.

Also @bmhan12 @kennyweiss @white238 for awareness

[adrienbernede]

@chapman39 this is moving in the opposite direction than #1402, as discussed in llnl/axom#1726 (comment).

It was a conscious change in #2558 (see rocm_path in axom package).

I think because this setting is shared in CachedCMakePackage it may not have shown up in your searches, but merging the current PR would in fact result in inconsistent requirements between several packages regarding the llvm-amdgpu prefix.

My experiments have told me that Spack was able to locate correctly llvm-amdgpu from the rocm prefix.

@alecbcs initially required that change to be consistent with spack installed rocm software stack. See #1402 (comment).

Paradoxically, your change in #2365 looks good to me (and to @alecbcs) because it removes the llvm suffix.

[chapman39]

@adrienbernede okay. thanks for the info. I will re-attempt to build Axom without this change, and set the prefix to prefix: /opt/rocm-x.y.z format, and will get back to you.