With LLVM main, which has the necessary changes to ld.lld and llvm-objcopy to support s390, and linux-next, which has the necessary changes to the kernel to support these tools, ARCH=s390 builds cleanly with LLVM=1. Cut builds that satisfy these conditions over to LLVM=1 so that regressions with this working setup do not occur.

Link: llvm/llvm-project@ fe3406e
Link: llvm/llvm-project@ 3c02cb7
Link: ClangBuiltLinux/linux#859
Link: ClangBuiltLinux/linux#1991
Link: ClangBuiltLinux/linux#1992
Link: ClangBuiltLinux/linux#1993
Link: ClangBuiltLinux/linux#1994
Link: ClangBuiltLinux/linux#1995
Link: ClangBuiltLinux/linux#1996

NOTE: linux-next as of next-20240221 does not contain the fix for llvm-objdump yet but as far as I am aware, that is only a warning so this should be safe to merge in its current state, but we can also wait for https://lore.kernel.org/linux-s390/20240220-s390-work-around-llvm-objdump-t-j-v1-1-47bb0366a831@kernel.org/ to appear in -next first.