[clang] [llvm] Revert changes in AddDefaultGCCPrefixes() for SystemZTriples. (PR #94729)

[Ilya Leoshkevich via cfe-commits]

iii-i wrote:

Looking at the code, this is because `Generic_GCC::GCCInstallationDetector::init()` will not try to strip any vendor except `unknown`. This will in turn cause `s390x-ibm-linux-gnu` to not detect GCC installation with prefix `s390x-linux-gnu`, which Debian and Ubuntu have. On the other hand, precisely for the same reason, `s390x-unknown-linux-gnu` works. Therefore I propose in #95407 to use that.

As to why the code written this way now, I don't have a good explanation, but, according to the comments in this PR, this looks intentional.

https://github.com/llvm/llvm-project/pull/94729