[clang] [Driver] Have getTargetSubDirPath better match get_compiler_rt_target (PR #100091)

[Rainer Orth via cfe-commits]

rorth wrote:

> Do we still need to fix this for 19.1.0-final?

I think so, yes.  E.g. when building on LLVM without specifying `LLVM_HOST_TRIPLE` explicitly (like when using `test-release.sh`), the `config.guess` output is used which is `sparc64-unknown-linux-gnu`.  However, `compiler-rt` always stores the 64-bit SPARC runtime libs in a `sparcv9-*` subdir, while `clang` only searches the target triple as determined or given.  So we have a inconsistency and every attempt to link a runtime lib fails.  Not a particularly satisfying user experience.

The situation on Solaris/amd64 is similar.

https://github.com/llvm/llvm-project/pull/100091