[CUDA] Add sm_121/Blackwell to known target (#24523)

## Summary
Add initial CUDA known target support for `sm_121` / Blackwell NVIDIA
GB10.

The CUDA execution limits are based on local cudaDeviceProp results from
an sm_121 device. Existing NVIDIA MMA ops are reused as a conservative
baseline until Blackwell-specific MMA intrinsics are modeled.

Related to #24477.
#24477 reports that IREE does not currently recognize newer Blackwell
CUDA targets such as `sm_120`. This PR addresses the same
target-enablement path for `sm_121`, which is the Blackwell target I can
validate locally on NVIDIA GB10.
It intentionally does not add `sm_120` support because I do not have
`sm_120` hardware to confirm the device limits or runtime behavior.

## Testing
Tested locally on NVIDIA GB10 / `sm_121`. `sm_121` requires PTX 8.8.
Using `+ptx88` compiles successfully.

Compiled and ran a local abs.mlir smoke test:
```
../iree-build/tools/iree-compile abs.mlir \
  --iree-hal-target-device=cuda \
  --iree-cuda-target=sm_121 \
  --iree-cuda-target-features=+ptx88 \
  -o abs_cuda.vmfb

../iree-build/tools/iree-run-module \
  --device=cuda \
  --module=abs_cuda.vmfb \
  --function=abs \
  --input=4xf32=-1,-2,3,-4
```

Results:
```
4xf32=1 2 3 4
```

Compiled and ran a local matmul.mlir smoke test:
```
../iree-build/tools/iree-compile matmul.mlir \
  --iree-hal-target-device=cuda \
  --iree-cuda-target=sm_121 \
  --iree-cuda-target-features=+ptx88 \
  -o matmul_cuda.vmfb

../iree-build/tools/iree-run-module \
  --device=cuda \
  --module=matmul_cuda.vmfb \
  --function=matmul \
  --input=128x256xf16=1 \
  --input=256x128xf16=1
```
Result: 128x128xf32 values are 256 as expected.

---------

Signed-off-by: Charlie-Tsai1123 <charlie1123tsai@gmail.com>