Auto merge of #132514 - Zalathar:print-target-cpus, r=jieyouxu

Port most of `--print=target-cpus` to Rust

The logic and formatting needed by `--print=target-cpus` has historically been carried out in C++ code. Originally it used `printf` to write directly to the console, but later it switched over to writing to a `std::ostringstream` and then passing its buffer to a callback function pointer.

This PR replaces that C++ code with a very simple function that writes a list of CPU names to a `&RustString`, with the rest of the logic and formatting being handled by ordinary safe Rust code.

Author: bors