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

[Fangrui Song via cfe-commits]

MaskRay wrote:

> `clang --version Target: s390x-ibm-linux`

What's `gcc -dumpmachine`? Is that `s390x-linux-gnu`. 

If the normalized default target triple `s390x-ibm-linux`, it's not supposed to detect a GCC installation for `s390x-linux-gnu`. You probably need to check your LLVM_DEFAULT_TARGET_TRIPLE config and set it to the normalized GCC target triple (arch-vendor-os-env instead of Debian multiarch style arch-os-env)

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