Don't commit thread stack on Windows

[upsuper]

On Windows, there is a system level resource limitation called commit limit, which is roughly the sum of physical memory + paging files[1]. CreateThread by default commits the stack size[2], which unnecessarily takes such resource from the shared limit.

This PR changes it to only reserve the stack size rather than commit it. Reserved memory would only take the address space of the current process until it's actually accessed.

This should make the behavior on Windows match other platforms, and is also a pretty standard practice on Windows nowadays.

[1] https://blogs.technet.microsoft.com/markrussinovich/2008/11/17/pushing-the-limits-of-windows-virtual-memory/
[2] https://docs.microsoft.com/zh-cn/windows/desktop/api/processthreadsapi/nf-processthreadsapi-createthread

[rust-highfive]

r? @sfackler

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[upsuper]

I have verified locally with the following program:

use std::thread;
use std::time::Duration;

fn main() {
    let threads: Vec<_> = (0..10).map(|_| {
        thread::Builder::new()
            .stack_size(100 * 1024 * 1024)
            .spawn(|| {
                thread::sleep(Duration::from_secs(3600));
            }).unwrap()
    }).collect();
    for thread in threads {
        thread.join().unwrap();
    }
}

It normally commits ~1GB memory, while building with a toolchain with this change, it only commits ~1MB.

[sfackler]

It looks like adding this flag is a pretty unambiguously the right thing to do:

https://bugs.chromium.org/p/webrtc/issues/detail?id=2902
https://bugs.java.com/bugdatabase/view_bug.do?bug_id=6316878
https://bugzilla.mozilla.org/show_bug.cgi?id=958796

cc @alexcrichton LGTY?

[alexcrichton]

@bors: r+

This is an awesome find! Sounds like a great idea to me

[bors]

📌 Commit fc8bb9c has been approved by alexcrichton

[bors]

⌛ Testing commit fc8bb9c with merge db54765...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing db54765 to master...