message: Changes default arch string for toss_4_cray specs
  Default is for tioga/rzvernal rather than rznevada.
  Also updates comment.
diff: "toss_4_x86_64_ib":
  [ "clang@14.0.0~openmp+rocm+mfem+c2c amdgpu_target=gfx906 ^raja~openmp+rocm ^umpire~openmp+rocm" ],
  - "__comment__":"# Use amdgpu_target=gfx90a for rzvernal ",
  + "__comment__":"# Use amdgpu_target=gfx90a for tioga/rzvernal; and gfx908 for rznevada",
  "toss_4_x86_64_ib_cray":
  - [ "clang@14.0.0~openmp+rocm+mfem+c2c amdgpu_target=gfx908 ^raja~openmp+rocm ^umpire~openmp+rocm" ],
  + [ "clang@14.0.0~openmp+rocm+mfem+c2c amdgpu_target=gfx90a ^raja~openmp+rocm ^umpire~openmp+rocm" ],
  "blueos_3_ppc64le_ib_p9":
  [ "clang@9.0.0~openmp+devtools+mfem+c2c",
  