Rustbuild on 32-bit MSVC fails run-make tests on AppVeyor

[alexcrichton]

In #37817 I discovered that enabling rustbuild fails to run 32-bit MSVC tests on AppVeyor. Specifically the run-make suite is failing with a bunch of errors like:

[02:33:05] C:/Program Files (x86)/Microsoft Visual Studio 14.0/VC/bin/amd64_x86/cl.exe: error while loading shared libraries: mspdb140.dll: cannot open shared object file: No such file or directory

I know I've seen this before, just want to open an issue to track this. I think this is something like we're adding the wrong directory to PATH or we're forgetting a directory entirely.

[retep998]

The tools in C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\bin\amd64_x86 are cross tools from a host of amd64 to a target of x86. This means that the native host directory needs to be in PATH specifically C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\bin\amd64. rustc does this just fine, but Rustbuild seems to be getting it wrong.