[fastcall] Fix null deref in concurrent access to the simulator

This CL fixes a null dereference when an attempt is made to access
the current arm64 simulator from a background thread.

Bug: chromium:1267491
Change-Id: I9232fe134fccbff162eb5076aff20884872e4cc7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3264219
Auto-Submit: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77757}

src/codegen/external-reference.cc

@@ -184,10 +184,8 @@ ExternalReference ExternalReference::Create(
     Isolate* isolate, ApiFunction* fun, Type type, Address* c_functions,
     const CFunctionInfo* const* c_signatures, unsigned num_functions) {
 #ifdef USE_SIMULATOR_WITH_GENERIC_C_CALLS
-  isolate->CurrentPerIsolateThreadData()
-      ->simulator()
-      ->RegisterFunctionsAndSignatures(c_functions, c_signatures,
-                                       num_functions);
+  Simulator::current(isolate)->RegisterFunctionsAndSignatures(
+      c_functions, c_signatures, num_functions);
 #endif  //  USE_SIMULATOR_WITH_GENERIC_C_CALLS
   return ExternalReference(Redirect(fun->address(), type));
 }

test/cctest/compiler/test-run-calls-to-external-references.cc

@@ -424,9 +424,7 @@ MachineType machine_type = MachineType::Int64();
 
 #define IF_SIMULATOR_ADD_SIGNATURE                                    \
   EncodedCSignature sig = m.call_descriptor()->ToEncodedCSignature(); \
-  m.main_isolate()                                                    \
-      ->CurrentPerIsolateThreadData()                                 \
-      ->simulator()                                                   \
+  Simulator::current(m.main_isolate())                                \
       ->AddSignatureForTargetForTesting(func_address, sig);
 #else  // def USE_SIMULATOR_WITH_GENERIC_C_CALLS
 #define IF_SIMULATOR_ADD_SIGNATURE

test/cctest/compiler/test-run-machops.cc

@@ -6706,9 +6706,7 @@ TEST(RunCallCFunction9) {
 #ifdef USE_SIMULATOR_WITH_GENERIC_C_CALLS
 #define IF_SIMULATOR_ADD_SIGNATURE                                    \
   EncodedCSignature sig = m.call_descriptor()->ToEncodedCSignature(); \
-  m.main_isolate()                                                    \
-      ->CurrentPerIsolateThreadData()                                 \
-      ->simulator()                                                   \
+  Simulator::current(m.main_isolate())                                \
       ->AddSignatureForTargetForTesting(func_address, sig);
 #else
 #define IF_SIMULATOR_ADD_SIGNATURE