[vm] Grab read program lock when querying Class functions.

Fixes https://github.com/dart-lang/sdk/issues/61734
TEST=tsan ci

Change-Id: Id0c905109ad88f9a0e4abc302b7ad90b8239a86a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/457140
Commit-Queue: Alexander Aprelev <aam@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>

runtime/vm/compiler/backend/flow_graph_compiler.cc

@@ -2013,6 +2013,7 @@ bool FlowGraphCompiler::LookupMethodFor(int class_id,
   Class& cls = Class::Handle(zone, raw_class);
   if (cls.IsNull()) return false;
   if (!cls.is_finalized()) return false;
+  SafepointReadRwLocker ml(thread, thread->isolate_group()->program_lock());
   if (Array::Handle(cls.current_functions()).IsNull()) return false;
 
   if (class_is_abstract_return != nullptr) {