Fix .NET wrapper PE Machine field: add PlatformTarget AnyCPU to prevent x64-only DLL

The managed Microsoft.Z3.dll was being built with Machine=0x8664 (AMD64)
because the Windows x64 CI environment (vcvarsall.bat x64) sets the MSBuild
Platform=x64 environment variable, which causes dotnet build to emit an x64
PE header even though the assembly is pure IL (ILONLY=True).

Fix: explicitly set <PlatformTarget>AnyCPU</PlatformTarget> in both build paths:
- scripts/mk_util.py: csproj template used by Python build system
  (mk_win_dist.py, mk_unix_dist.py)
- src/api/dotnet/Microsoft.Z3.csproj.in: csproj template used by CMake build

This produces Machine=0x014C (i386/AnyCPU) so arm64 .NET hosts on macOS,
Linux and Windows can load the wrapper without error FS0193.

Closes #9863

scripts/mk_util.py

@@ -1804,6 +1804,7 @@ class DotNetDLLComponent(Component):
     <Description>Z3 is a satisfiability modulo theories solver from Microsoft Research.</Description>
     <Copyright>Copyright Microsoft Corporation. All rights reserved.</Copyright>
     <PackageTags>smt constraint solver theorem prover</PackageTags>
+    <PlatformTarget>AnyCPU</PlatformTarget>
     %s
   </PropertyGroup>
 

src/api/dotnet/Microsoft.Z3.csproj.in

@@ -60,6 +60,7 @@
     <Warn>4</Warn>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <DocumentationFile>$(OutputPath)\Microsoft.Z3.xml</DocumentationFile>
+    <PlatformTarget>AnyCPU</PlatformTarget>
   </PropertyGroup>
 
   <!-- Compilation items -->