Rollup merge of #72133 - bdbai:master, r=joshtriplett

Add target thumbv7a-uwp-windows-msvc

Add target spec for thumbv7a-uwp-windows-msvc, so that libraries written in Rust will have a chance to run on ARM-based devices with Windows 10.

So far I managed to create a proof-of-concept library for Universal Windows Platform apps to consume and it worked on a Windows Phone. However, building a standalone executable seemed troublesome due to `LLVM ERROR: target does not implement codeview register mapping` stuff (see also https://github.com/rust-lang/rust/issues/52659#issuecomment-408233322 ).

Steps to test:
1. Clone and build this version
```sh
git clone https://github.com/bdbai/rust.git
cd rust
python x.py build -i --target thumbv7a-uwp-windows-msvc --stage 1 src/libstd
rustup toolchain link arm-uwp-stage1 .\build\x86_64-pc-windows-msvc\stage1\
```
2. Create a new library crate
```sh
cargo new --lib arm-uwp-test
cd arm-uwp-test
```
3. Change `crate-type` in `Cargo.toml` to `staticlib`
```toml
[lib]
crate-type=["staticlib"]
```
4. Replace the following code in `src/lib.rs`
```rust
#[no_mangle]
pub extern "system" fn call_rust() -> i32 {
    2333
}
```
5. Build the crate
```sh
cargo +arm-uwp-stage1 build -v --target thumbv7a-uwp-windows-msvc
```
6. `arm-uwp-test.lib` should appear in `target\thumbv7a-uwp-windows-msvc\debug`

To consume this library:
1. Make sure Visual Studio 2017 and Windows 10 SDK (10.0.17134 or above) are installed
2. Create a new Blank App (C++/WinRT) in Visual Studio 2017 (Visual Studio 2019 cannot deploy UWP apps to Windows Phone)
3. Go to Property Pages, and then Linker->Input->Additional Dependencies, add `arm-uwp-test.lib` produced just now
4. Manually declare function prototypes in `MainPage.h`
```c++
extern "C" {
    int call_rust();
}
```
5. Replace the `ClickHandler` part in `MainPage.cpp`
```c++
myButton().Content(box_value(std::to_wstring(call_rust())));
```
6. Build and deploy this app to an ARM device running Windows 10. The app should run and show `2333` when the button is clicked.

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