Avoid building JITs for cross tools when unnecessary (#116332)

Makes the builds that happen in the SPMI pipelines a bit faster. Maybe
helps in some other cases too.

Author: jakobbotsch

eng/Subsets.props

@@ -358,8 +358,8 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <!-- When building cross components, always build the JITs. We will need them for running crossgen2 and ILC in the build. -->
-    <_CrossToolSubset Condition="'$(_BuildCrossComponents)' == 'true'" Include="ClrAllJitsSubset=true" />
+    <!-- crossgen2/ILC have dependencies on the JITs, so build them if the cross component includes crossgen2/ILC. -->
+    <_CrossToolSubset Condition="'$(_BuildCrossComponents)' == 'true' and ($(_subset.Contains('+clr.tools+')) or $(_subset.Contains('+clr.nativecorelib+')) or $(_subset.Contains('+clr.crossarchtools+')))" Include="ClrAllJitsSubset=true" />
     <!-- When targeting Windows, we need to build a copy of the diagnostic libraries that can run on the host to enable remote debugging of a local build. -->
     <_CrossToolSubset Condition="'$(_BuildCrossComponents)' == 'true' and '$(TargetsWindows)' == 'true'" Include="ClrDebugSubset=true" />
     <!--