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

[Fangrui Song via cfe-commits]

MaskRay wrote:

This is fine as a temporary workaround. What's `clang --version` on Ubuntu s390x?
If it's390x-unknown-linux-gnu`, we have code to use `lib/s390x-linux-gnu`. CMake LLVM_DEFAULT_TARGET_TRIPLE can be used to set the default target triple.

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