Make llvm path consistent between llvm and llvm-amdgpu in enzyme pkg

[chapman39]

This PR makes the llvm pathing consistent between llvm and llvm-amdgpu. This way, specifying llvm-amdgpu in the spack.yaml will look like the following.

    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

[chapman39]

@v-dobrev

[alecbcs]

@tgymnich @vchuravy @wsmoses @amd-toolchain-support any objections here?
It would be a breaking change to users' configuration, but it makes a lot of sense to me.

[alecbcs]

Looks good to me. Thank you @chapman39!