Rollup merge of #105458 - Ayush1325:blocking_spawn, r=Mark-Simulacrum

Rollup merge of #105458 - Ayush1325:blocking_spawn, r=Mark-Simulacrum

Allow blocking `Command::output`

### Problem
Currently, `Command::output` is internally implemented using `Command::spawn`. This is problematic because some targets (like UEFI) do not actually support multitasking and thus block while the program is executing. This coupling does not make much sense as `Command::output` is supposed to block until the execution is complete anyway and thus does not need to rely on a non-blocking `Child` or any other intermediate.

### Solution
This PR moves the implementation of `Command::output` to `std::sys`. This means targets can choose to implement only `Command::output` without having to implement `Command::spawn`.

### Additional Information

This was originally conceived when working on https://github.com/rust-lang/rust/pull/100316. Currently, the only target I know about that will benefit from this change is UEFI.

This PR can also be used to implement more efficient `Command::output` since the intermediate `Process` is not actually needed anymore, but that is outside the scope of this PR.

Since this is not a public API change, I'm not sure if an RFC is needed or not.