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

[Chen Zheng via cfe-commits]

https://github.com/chenzheng1030 commented:

We still miss many implications in `PPCTargetInfo::setFeatureEnabled()`. This patch fixes the implications for `-msoft-float`. But implications like `-mno-power8-altivec` for `-crypto` is still not handled.

Let's leave them for now and make this patch focus on `-msoft-float`. There will be a refactor for PPC target for the target features based on `PPCTargetParser.cpp`. I think that time is a better time to handle all other feature implications.

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