[VM runtime] Dual mapping of executable pages.

Change-Id: Iaad78d324e25462ce951f4df26974a6a368c50b7
Reviewed-on: https://dart-review.googlesource.com/c/93377
Commit-Queue: Régis Crelier <regis@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>

runtime/vm/code_patcher.h

@@ -19,10 +19,12 @@ class RawCode;
 class RawFunction;
 class RawObject;
 
+#if defined(TARGET_ARCH_IA32) || defined(TARGET_ARCH_DBC)
 // Stack-allocated class to create a scope where the specified region
 // [address, address + size] has write access enabled. This is used
 // when patching generated code. Access is reset to read-execute in
 // the destructor of this scope.
+// Dual mapping of instructions pages is not supported on these target arch.
 class WritableInstructionsScope : public ValueObject {
  public:
   WritableInstructionsScope(uword address, intptr_t size);
@@ -32,6 +34,7 @@ class WritableInstructionsScope : public ValueObject {
   const uword address_;
   const intptr_t size_;
 };
+#endif  // defined(TARGET_ARCH_IA32) || defined(TARGET_ARCH_DBC)
 
 class CodePatcher : public AllStatic {
  public: