Fix code version table lock / coop GC mode switch ordering (#25129)

Fix code version table lock / coop GC mode switch ordering (#25129)

Fixes https://github.com/dotnet/coreclr/issues/25086
- The lock is taken inside other unsafe locks in coop mode, in some paths it may not be legal to switch to preemptive mode before taking the lock
- Iterating over the cross loader allocator hash table to backpatch entry point slots needs coop GC mode, and was being done inside the lock
- Moved the switch to coop GC mode to before acquiring the lock to maintain consistent ordering between the two