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: llvm-amdgpu@6.4.2
        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