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

• Closes release build failure after Implement Default Terminal #7489
• I work here.
• 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.