[clang] [Driver][Sparc] Default to -mcpu=v9 for 32-bit Linux/sparc64 (PR #109278)

[Fangrui Song via cfe-commits]

MaskRay wrote:

We try to restrict distribution differences to things like default linker options and library paths. Affecting `-mcpu=` seems very unintuitive. There are many Debian derivatives. It's weird that Debian uses -mcpu=v9 while others use -mcpu=v8. We should not increase `IsDebian` or `IsGentoo` use.

If -mcpu=v9 seems the right thing for the majority of configurations, we can bump -mcpu=v9 for all Linux and ask other, older systems to use https://clang.llvm.org/docs/UsersManual.html#configuration-files to specify `-mcpu=v8`.

https://github.com/llvm/llvm-project/pull/109278