dotnet: force PlatformTarget=AnyCPU to fix arm64 host load failure (#9868)

`Microsoft.Z3.dll` was being emitted with PE `Machine=0x8664` (AMD64)
despite being pure IL, because the Windows x64 CI environment
(`vcvarsall.bat x64`) injects `Platform=x64` into MSBuild, which sets
the COFF machine type when `PlatformTarget` is not explicitly specified.
This causes `FS0193` / architecture mismatch errors on any arm64 .NET
host (macOS Apple Silicon, Linux aarch64, Windows on ARM).

## Changes

- **`scripts/mk_util.py`** — Add
`<PlatformTarget>AnyCPU</PlatformTarget>` to the in-memory `.csproj`
template generated by the Python build system (`mk_win_dist.py`,
`mk_unix_dist.py`)
- **`src/api/dotnet/Microsoft.Z3.csproj.in`** — Add
`<PlatformTarget>AnyCPU</PlatformTarget>` to the CMake build system
template

Both paths now produce `Machine=0x014C` (i386/AnyCPU) regardless of host
or CI environment, matching the assembly's actual nature (`ILONLY=True`,
`32BIT_REQUIRED=False`). The native side already ships six RIDs; no
changes needed there.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

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 -->