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

[Fangrui Song via cfe-commits]

MaskRay wrote:

> gcc -dumpmachine s390x-linux-gnu

Then I am afraid your clang configuration is not correct. It previously detecting the GCC installation at `s390x-linux-gnu` is a mistake. You need to set LLVM_DEFAULT_TARGET_TRIPLE correctly.

If for some reason you cannot change that, add `--gcc-triple=s390x-linux-gnu` to the file `bin/s390x-ibm-linux.cfg` beside `bin/clang`. You probably also need `-march=xxx`.

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