[libclc] [libclc] Disable external-calls testing for clspv targets (PR #127529)

[Fraser Cormack via cfe-commits]

frasercrmck wrote:

Just one failure:

```
********************
Timed Out Tests (1):
  MLIR :: Examples/standalone/test.toy

Total Discovered Tests: 131804
  Skipped          :     48 (0.04%)
  Unsupported      :   2855 (2.17%)
  Passed           : 128580 (97.55%)
  Expectedly Failed:    320 (0.24%)
  Timed Out        :      1 (0.00%)
```

I don't think this is related.

https://github.com/llvm/llvm-project/pull/127529