[clang] [PPC] Disable vsx and altivec when -msoft-float is used (PR #100450)

[Chen Zheng via cfe-commits]

https://github.com/chenzheng1030 commented:

```
def FeatureFPU       : SubtargetFeature<"fpu","HasFPU","true",
                                        "Enable classic FPU instructions",
                                        [FeatureHardFloat]>;
def FeatureAltivec   : SubtargetFeature<"altivec","HasAltivec", "true",
                                        "Enable Altivec instructions",
                                        [FeatureFPU]>;
```

If `FeatureHardFloat` is false, `FeatureFPU` should be false and `FeatureAltivec` should be false too.

Seems we are lacking more handling in `PPCTargetInfo::setFeatureEnabled()`, for example, with this fix, -msoft-float disables vsx and altivec, but it does not disable `direct-move`, `power8-vector`....

https://github.com/llvm/llvm-project/pull/100450