Remove support for older LLVM versions, and re-order linker flags

[directhex]

We have generally tried to support linking multiple versions of LLVM within our git tree. Every new LLVM version moves symbols around between libraries, and as a result, every new version of LLVM requires different linker flags to build. The command line tool llvm-config should tell you the exact flags you need, but it is a problem for us when cross-compiling to rely on this, and as a result, we transcribe the result of various llvm-config outputs directly into Mono's CMakeLists.txt.

In an effort to support multiple versions of LLVM, flags common between all supported versions were kept in one place, then the version-specific flags appended afterwards. And this has worked fine for years. However:

1. Whilst we only link with lld, it is common for contributors and source-build to link with gold, bfd, or some other GNU-flavoured linker, where library order is essential
2. The list of common libraries to link has remained unchanged for years, but the symbol intra-dependencies may have changed a long time ago, so common symbol order cannot be assumed to remain valid between LLVM versions

This has resulted in a long-standing problem for people using e.g. Debian or Ubuntu or GitHub CodeSpaces, rather than always building with one of our dockerfiles representing our "real" build environment, when targeting platforms which use Mono and link LLVM.

[directhex]

It looks like the linker order between LLVMIRReader and LLVMObject got codified in d2939d3 3 years ago. I'd need to find ancient artifacts to confirm, but my suspicion is, with LLVM 11, LLVMObject did not have a hard dependency on LLVMIRReader and thus it was fine to have the linker order put LLVMIRReader before LLVMObject (GNU linkers need the symbol consumer listed before the symbol provider). And since those libraries still existed in LLVM 12+, and the old linker order still worked (as we build with LLVM lld, which cares less about library order), we didn't notice that maintaining the old linker order forever is actually harmful.

We need to regenerate the full linker list in src/mono/CMakeLists.txt using the relevant llvm-config for every major LLVM update.

[carlossanlop]

Let's run the other azp runs that are most likely affected. Just for peace of mind.

[carlossanlop]

/azp run runtime-wasm

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[kg]

I tested building with this PR and it appears to resolve the issues I was having on Debian Bookworm with its system packages (llvm/clang 14)

[ilonatommy]

Closing and opening to re-trigger the CI after the outage

[directhex]

Okay the commit history is a mess, but I updated the linkages to LLVM 19. Ignore the branch name.