Auto merge of rust-lang#146700 - Zalathar:quoted-args, r=nikic

cg_llvm: Move target machine command-line quoting from C++ to Rust

When this code was introduced in rust-lang#130446 and rust-lang#131805, it was complicated by the need to maintain compatibility with earlier versions of LLVM.

Now that LLVM 20 is the baseline (rust-lang#145071), we can do all of the quoting in pure Rust code, and pass two flat strings to LLVM to be used as-is.

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In this PR, my priority has been to preserve the existing behaviour as much as possible, without worrying too much about what the behaviour *should* be. (Though I did avoid a leading space before the first argument.)

Author: bors