Bug 1786774 - Make the memory allocator spinlocks always spin in kernel-space on macOS/AArch64. r=gsvelto

Differential Revision: https://phabricator.services.mozilla.com/D169067

memory/build/Mutex.cpp

@@ -8,14 +8,18 @@
 
 // static
 bool Mutex::SpinInKernelSpace() {
+#  ifdef __aarch64__
+  return true;
+#  else
   if (__builtin_available(macOS 10.15, *)) {
     return true;
   }
 
   return false;
+#  endif
 }
 
 // static
-bool Mutex::gSpinInKernelSpace = SpinInKernelSpace();
+const bool Mutex::gSpinInKernelSpace = SpinInKernelSpace();
 
 #endif  // defined(XP_DARWIN)

memory/build/Mutex.h

@@ -136,7 +136,7 @@ struct MOZ_CAPABILITY("mutex") Mutex {
 
 #if defined(XP_DARWIN)
   static bool SpinInKernelSpace();
-  static bool gSpinInKernelSpace;
+  static const bool gSpinInKernelSpace;
 #endif  // XP_DARWIN
 };