Disable IBCMerge on OSX builds (dotnet#33393)

* Disable System.Private.CoreLib IBCMerge on MacOS.

* Disable IBCMerge on mac across the board.

* Update comment to include link to issue.

eng/codeOptimization.targets

@@ -4,6 +4,12 @@
      <IsEligibleForNgenOptimization>true</IsEligibleForNgenOptimization>
      <IsEligibleForNgenOptimization Condition="'$(IsReferenceAssembly)' == 'true'">false</IsEligibleForNgenOptimization>
      <IsEligibleForNgenOptimization Condition="'$(GeneratePlatformNotSupportedAssembly)' == 'true' or '$(GeneratePlatformNotSupportedAssemblyMessage)' != ''">false</IsEligibleForNgenOptimization>
+      <!-- There's an issue causing IBCMerge failures because of mismatched MVIDs
+           across many of our assemblies on Mac, so disable
+           IBCMerge optimizations on Mac for now to unblock the offical build.
+           See issue https://github.com/dotnet/runtime/issues/33303
+      -->
+      <IsEligibleForNgenOptimization Condition="'$(TargetOS)' == 'OSX'">false</IsEligibleForNgenOptimization>
   </PropertyGroup>
 
   <Target Name="SetApplyNgenOptimization"