[release/6.0] Support platforms where crossgen2 is not available (#36325)

* Do not use crossgen2 when targeting s390x (where it is not supported)

* Only reference "Microsoft.NETCore.App.Runtime.$(RuntimeIdentifier)"
  and "Microsoft.NETCore.App.Crossgen2.$(BuildOsName)-$(Crossgen2BuildArchitecture)"
  when actually using crossgen2.

src/Framework/App.Runtime/src/Microsoft.AspNetCore.App.Runtime.csproj

@@ -42,7 +42,8 @@ This package is an internal implementation of the .NET Core SDK and is not meant
     <!-- Pack .ni.r2rmap files in symbols package (native symbols for Linux) -->
     <AllowedOutputExtensionsInSymbolsPackageBuildOutputFolder>$(AllowedOutputExtensionsInSymbolsPackageBuildOutputFolder);.r2rmap</AllowedOutputExtensionsInSymbolsPackageBuildOutputFolder>
 
-    <!-- Optimize the framework using the crossgen2 tool -->
+    <!-- Optimize the framework using the crossgen2 tool. Crossgen2 is not currently supported on s390x. -->
+    <CrossgenOutput Condition=" '$(TargetArchitecture)' == 's390x' ">false</CrossgenOutput>
     <CrossgenOutput Condition=" '$(CrossgenOutput)' == '' AND '$(Configuration)' != 'Debug' ">true</CrossgenOutput>
 
     <!-- Produce crossgen2 profiling symbols (.ni.pdb or .r2rmap files). -->
@@ -120,7 +121,8 @@ This package is an internal implementation of the .NET Core SDK and is not meant
       <IncludeAssets>Runtime;Native</IncludeAssets>
     </Reference>
 
-    <Reference Include="Microsoft.NETCore.App.Runtime.$(RuntimeIdentifier)"
+    <Reference Condition="'$(CrossgenOutput)' == 'true'"
+        Include="Microsoft.NETCore.App.Runtime.$(RuntimeIdentifier)"
         ExcludeAssets="All"
         PrivateAssets="All"
         GeneratePathProperty="true" />
@@ -129,7 +131,8 @@ This package is an internal implementation of the .NET Core SDK and is not meant
       This package contains the crossgen2 tool. Unfortunately, it doesn't make the tool easy to use.
       $(GeneratePathProperty) and hacks in the _ExpandRuntimePackageRoot target work around the gaps.
     -->
-    <Reference Include="Microsoft.NETCore.App.Crossgen2.$(BuildOsName)-$(Crossgen2BuildArchitecture)"
+    <Reference Condition="'$(CrossgenOutput)' == 'true'"
+        Include="Microsoft.NETCore.App.Crossgen2.$(BuildOsName)-$(Crossgen2BuildArchitecture)"
         ExcludeAssets="All"
         PrivateAssets="All"
         GeneratePathProperty="true" />