Fix ARM64 build by defining WIN32 (#9654)

By default from ARM64 architecture projects, `WIN32` is not defined. It
is supposed to be for this proxy stub to work. So I've set it with the
preprocessor for this project.

## PR Checklist
* [x] Closes release build failure after #7489 
* [x] I work here.
* [x] Built on my machine.

## Detailed Description of the Pull Request / Additional comments

`WIN32` appears to convey two meanings depending on who you are:
- To most of Windows, `WIN32` appears to mean the Win32 API surface and
  sometimes the major OS version that goes with it. (Specifically in
  contrast to 16-bit Windows.)
- To others, `WIN32` appears to mean a 32-bit processor or a synonym of
  `x86`.

This is generally not a problem for a few reasons:
- VS defines `WIN32` in the default targets/props only for the `x86`
  processor type. **BUT**
- Windows defines `WIN32` if it's not already defined in both
  `minwinbase.h` and `ole2.h` which generally speaking manage to get
  compiled into practically everything especially since `minwinbase.h`
  tends to sneak itself in somehow through `windows.h` and that's
  **THE** include to use the Windows API surface.
- Windows also defines `WIN32` for itself unconditionally and relatively
  globally when building itself.

However, it's a problem here because: 
- `rpcproxy.h` is the only header included in `dlldata.c`, a file
  generated automatically by `midl.exe` in the SDK when making a proxy
  stub.
- `rpcproxy.h` only defines its contents for a proxy when `WIN32` or
  `_M_AMD64` are found.
- Therefore, it's defined pretty naturally for x86 and AMD64 targets
  from VS, but not for ARM64.
- ARM64 support is pretty new and those who are attempting to build for
  ARM64 and against the public SDK with Visual Studio for a classic COM
  proxy... seems like a relatively unlikely combination.

I will follow up with the Visual Studio, Windows SDK, and MIDL/COM teams
to try to remove this pitfall from the public tooling. But for now, this
is the fix.

src/host/proxy/Host.Proxy.vcxproj

@@ -57,8 +57,9 @@
   <ItemDefinitionGroup>
     <ClCompile>
       <AdditionalIncludeDirectories>..;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
-      <CallingConvention>StdCall</CallingConvention> <!-- Must be Stdcall on all platforms to resolve _ObjectStublessClient3 -->
-      <PreprocessorDefinitions>REGISTER_PROXY_DLL;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <CallingConvention>StdCall</CallingConvention>
+      <!-- Must be Stdcall on all platforms to resolve _ObjectStublessClient3 -->
+      <PreprocessorDefinitions>REGISTER_PROXY_DLL;WIN32;%(PreprocessorDefinitions)</PreprocessorDefinitions>
       <PrecompiledHeader>NotUsing</PrecompiledHeader>
     </ClCompile>
     <Link>
@@ -67,4 +68,4 @@
   </ItemDefinitionGroup>
   <!-- Careful reordering these. Some default props (contained in these files) are order sensitive. -->
   <Import Project="$(SolutionDir)src\common.build.post.props" />
-</Project>
+</Project>