Rollup merge of #118378 - cormacrelf:bugfix/linker-plugin-lto-wasm, r=petrochenkov

Perform LTO optimisations with wasm-ld + -Clinker-plugin-lto

Fixes (partially) #60059. Technically, `--target wasm32-unknown-unknown -Clinker-plugin-lto` would complete without errors before, but it was not producing optimized code. At least, it may have been but it was probably not the opt-level people intended.

Similarly to #118377, this could benefit from a warning about using an explicit libLTO path with LLD, which will ignore it and use its internal LLVM. Especially given we always use lld on wasm targets. I left the code open to that possibility rather than making it perfectly neat.

Author: matthiaskrgr