Use ProcessArch instead of OSArch for 'executing host'

Author: am11

src/coreclr/nativeaot/BuildIntegration/Microsoft.DotNet.ILCompiler.SingleEntry.targets

@@ -15,7 +15,7 @@
     <!-- On non-Windows, determine _hostArchitecture from NETCoreSdkPortableRuntimeIdentifier -->
     <_hostArchitecture Condition="'$(OS)' != 'Windows_NT'">$(NETCoreSdkPortableRuntimeIdentifier.SubString($([MSBuild]::Add($(NETCoreSdkPortableRuntimeIdentifier.LastIndexOf('-')), 1))))</_hostArchitecture>
     <!-- On Windows, default to host that matches OS architecture on Windows. It allows compilation using msbuild.exe that is x86 by default -->
-    <_hostArchitecture Condition="'$(OS)' == 'Windows_NT'">$([System.Runtime.InteropServices.RuntimeInformation]::OSArchitecture.ToString().ToLowerInvariant)</_hostArchitecture>
+    <_hostArchitecture Condition="'$(OS)' == 'Windows_NT'">$([System.Runtime.InteropServices.RuntimeInformation]::ProcessArchitecture.ToString().ToLowerInvariant)</_hostArchitecture>
 
     <!-- Determine _targetArchitecture from RuntimeIdentifier -->
     <_targetArchitecture>$(RuntimeIdentifier.SubString($([MSBuild]::Add($(RuntimeIdentifier.LastIndexOf('-')), 1))))</_targetArchitecture>