Rollup merge of #156554 - guybedford:wasm-use-legacy-eh, r=alexcrichton

Allow user-provided `llvm_args` to override target spec arguments

This switches the order in which `-Cllvm-args` is applied between target-spec arguments and user-provided LLVM arguments.

This came up in rust-lang/rust#156061, where the target passing `-Cllvm-args=-wasm-use-legacy-eh=false` means that a user passing `-Cllvm-args=-wasm-use-legacy-eh=true` cannot override this value since the LLVM arguments support the last argument overriding the previous, and user arguments were chained first.

With this change, it is possible for Wasm targets to opt into legacy EH for compatibility with runtimes that don't yet implement the modern exnref/try_table instructions, such as Node.js 20 on V8 11.3 and older browsers. While Node.js 20 is formally EOL, many libraries will still need to support this version for a few months yet, so this would ease the transition path to modern exception handling having an opt-out.

Originally this PR added support for a dedicated `-Z` flag for switching to legacy exception handling, but fine-grained control over the arguments would be a preferable solution provided it does not conflict with other behaviours.

//cc @alexcrichton