Auto merge of #53621 - jordanrh1:windows-arm, r=alexcrichton

Auto merge of #53621 - jordanrh1:windows-arm, r=alexcrichton

Add target thumbv7a-pc-windows-msvc

This is an early draft of support for Windows/ARM. To test it,

1. Install Visual Studio 2017 and Windows SDK version 17134.
1. Obtain alexcrichton/xz2-rs#35, rust-lang-nursery/compiler-builtins#256, and the fix for [LLVM Bug 38620](https://bugs.llvm.org/show_bug.cgi?id=38620).
2. Open a command prompt and run
```
set CC_thumbv7a-pc-windows-msvc=C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\VC\Tools\MSVC\14.11.25503\bin\HostX64\arm\CL.exe
set CFLAGS_thumbv7a-pc-windows-msvc=/D_ARM_WINAPI_PARTITION_DESKTOP_SDK_AVAILABLE=1 /nologo
c:\python27\python.exe x.py build --host x86_64-pc-windows-msvc --build x86_64-pc-windows-msvc --target thumbv7a-pc-windows-msvc
```

It will build the stage 2 compiler, but fail building stage 2 test. To build an executable targeting windows/arm,
1. Copy `build\x86_64-pc-windows-msvc\stage0\bin\cargo.exe` to `build\x86_64-pc-windows-msvc\stage2\bin`
2. Open a command prompt and run
```
"C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\VC\Auxiliary\Build\vcvars64.bat"
set PATH=build\x86_64-pc-windows-msvc\stage2\bin;%PATH%
cargo new hello
cd hello
cargo build --target thumbv7a-pc-windows-msvc –release
```

Copy target\thumbv7a-pc-windows-msvc\release\hello.exe to your platform and run.

There are a number of open issues that I'm hoping to get help with:

 - Error when compiling the `test` crate: `error: cannot link together two panic runtimes: panic_abort and panic_unwind`
 - Warnings when building the compiler_builtins crate: `warning: cl : Command line warning D9002 : ignoring unknown option '-fvisibility=hidden'`. It looks like the build system is passing GCC-style flags to MSVC.
 - How to specify the LIBPATH entries for ARM. Right now they are hardcoded as absolute paths in the target spec.

This pull request depends on
 - alexcrichton/xz2-rs#35 - update vcxproj to Visual Studio 2017
 - rust-lang-nursery/compiler-builtins#256 - fix compile errors when building for windows/arm
 - [Bug 38620 - ARM: Incorrect COFF relocation type for thumb bl instruction](https://bugs.llvm.org/show_bug.cgi?id=38620)

This PR updates #52659