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

[Ulrich Weigand via cfe-commits]

uweigand wrote:

This `/etc/lsb-release` test looks a bit weird to me.  Why can't it simply do:
```
        echo ${UNAME_MACHINE}-ibm-linux-${LIBC}
```
like the upstream `config.guess` does?

That should properly match the other variants that are simply missing one of the triple fields.


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