VS backend has serious cross-compiling issues

Nobody has claimed this yet.

**Describe the bug**
When setting up cross-compiling, you're supposed to use `host_machine` to indicate what will be running the resulting binaries, and `target_machine` if you're building a compiler which might target a different machine. The VS2010 backend uses `target_machine` to populate the `.vcxproj` file's architecture, not `host_machine`.

What's worse is that the compiler binary that was pointed to during the VS solution generation step is completely ignored. So, if you just specify `cl.exe` as the compiler, then as long as there's *any* `cl.exe` in your path, that's what the generation step will use for running all of its tests. Then, after the project is created and you load it in VS, the `target_machine` you used ends up selecting which actual compiler will be used.

**To Reproduce**
Attempt to cross-compile with `--backend=vs2019` and a `--cross-file` pointing to something that looks like:
```
[host_machine]
system = 'windows'
cpu_family = 'aarch64'
cpu = 'aarch64'
endian = 'little'

[binaries]
c = 'cl'
cpp = 'cl'
c_ld = 'link'
cpp_ld = 'link'
cmake = 'cmake'
pkgconfig = 'pkg-config'
```

**Expected behavior**
Some form of error, because the `cl.exe` that gets invoked identifies itself as:
```
C++ compiler for the host machine: cl (msvc 19.28.29913 "Microsoft (R) C/C++ Optimizing Compiler Version 19.28.29913 for x64")
```
Which is wrong when the `host_machine` in the cross file is supposed to be arm64/aarch64.

**system parameters**
* Is this a [cross build](https://mesonbuild.com/Cross-compilation.html) or just a plain native build (for the same computer)?: Yes
* what operating system (e.g. MacOS Catalina, Windows 10, CentOS 8.0, Ubuntu 18.04, etc.): Windows 10
* what Python version are you using e.g. 3.8.0: 3.8.2
* what `meson --version`: 0.57.999
* what `ninja --version` if it's a Ninja build: N/A

Start by reproducing the issue with --backend=vs2019 and the shown --cross-file, then trace the VS backend's architecture selection and compiler discovery. Done means the backend respects host_machine for the generated project and does not silently ignore the configured compiler when validating the cross build.

Written by the indexing model from the issue text.