8319777: Zero: Support 8-byte cmpxchg

8320582: Zero: Misplaced CX8 enablement flag

Reviewed-by: phh
Backport-of: 6b96bb640aa91d96877b8ceea5fed359607c1e45

Author: shipilev

src/hotspot/cpu/zero/vm_version_zero.cpp

@@ -137,6 +137,12 @@ void VM_Version::initialize() {
 #ifdef ASSERT
   UNSUPPORTED_OPTION(CountCompiledCalls);
 #endif
+
+  // Supports 8-byte cmpxchg with compiler built-ins.
+  // These built-ins are supposed to be implemented on
+  // all platforms (even if not natively), so we claim
+  // the support unconditionally.
+  _supports_cx8 = true;
 }
 
 void VM_Version::initialize_cpu_information(void) {