RFC: Add blocking spawn for std::process::Command

Problem

All of the current functions in std::process::Command are based around a non-blocking spawn implementation. This is a problem for targets where it is possible to execute programs but no concept of processes or threads.

I am working on implementing Rust std for UEFI, which is one such case. To get a working implementation of std::process::Command, I had to make Command::spawn blocking (and wait till execution ends for the launched program). More about this can be found in the PR

Proposal

Add a blocking spawn/execute function in sys::{target}::process::Command and use this function for std::process::Command::output implementation. This should have minimal to no functionality impact but will help decouple Command::output from Command::spawn.