[clang] [Driver] Default to -mcpu=ultrasparc3 on Solaris/SPARC (PR #149990)

[Rainer Orth via cfe-commits]

rorth wrote:

I've got a follow-up patch to similarly default to `v8plus` on 32-bit Solaris/SPARC.  I wonder if this one or both would be appropriate for the `release/21.x` branch?

https://github.com/llvm/llvm-project/pull/149990