Test failures on PPC64 with GCC 15 (Power9 baseline)

Hi,

On the upcoming Ubuntu 25.10, when compiling and running highway using the Power9 baseline with GCC 15, we get the following failures:

```
The following tests FAILED:
405 - HwyRandomTestGroup/HwyRandomTest.TestSeeding/PPC9 # GetParam() = 281474976710656 (ILLEGAL)
406 - HwyRandomTestGroup/HwyRandomTest.TestMultiThreadSeeding/PPC9 # GetParam() = 281474976710656 (ILLEGAL)
407 - HwyRandomTestGroup/HwyRandomTest.TestRandomUint64/PPC9 # GetParam() = 281474976710656 (ILLEGAL)
408 - HwyRandomTestGroup/HwyRandomTest.TestNextNRandomUint64/PPC9 # GetParam() = 281474976710656 (ILLEGAL)
409 - HwyRandomTestGroup/HwyRandomTest.TestNextFixedNRandomUint64/PPC9 # GetParam() = 281474976710656 (ILLEGAL)
410 - HwyRandomTestGroup/HwyRandomTest.TestCachedXorshiro/PPC9 # GetParam() = 281474976710656 (ILLEGAL)
411 - HwyRandomTestGroup/HwyRandomTest.TestUniformDist/PPC9 # GetParam() = 281474976710656 (ILLEGAL)
412 - HwyRandomTestGroup/HwyRandomTest.TestNextNUniformDist/PPC9 # GetParam() = 281474976710656 (ILLEGAL)
413 - HwyRandomTestGroup/HwyRandomTest.TestNextFixedNUniformDist/PPC9 # GetParam() = 281474976710656 (ILLEGAL)
414 - HwyRandomTestGroup/HwyRandomTest.TestUniformCachedXorshiro/PPC9 # GetParam() = 281474976710656 (ILLEGAL)
```

Tests run fine if we either drop back to -mcpu=power8 or use GCC 14.

The issue appears both in our packaged version and in a fresh git clone.