[CUDA] Raise the default PTX level for sm_89, sm_120 and sm_121 (#24976)

Pick the default per architecture: 8.8 for sm_121, 8.7 for sm_120 and
8.4 for sm_89. An explicit --iree-cuda-target-features can still
override this.

The CUDA target always passes an explicit PTX level to the NVPTX backend
and only raised the +ptx76 default to 7.8 for sm_89. That is below what
the backend accepts for Blackwell, so `--iree-cuda-target=sm_120`
(minimum 8.7) and `sm_121` (minimum 8.8) failed for every program, and
below what the FP8 mma.sync intrinsics selected by default on Ada need,
so an f8E4M3FN matmul on sm_89 failed to select
`llvm.nvvm.mma.m16n8k32.row.col.f32.e4m3.e4m3.f32` (PTX 8.4).

Fixes #24959

Assisted-by: Claude Code

Signed-off-by: Samarth Narang <sn2910@nyu.edu>