A64: Synchronize with r16993.

src/platform.h

Index: src/platform.h
diff --git a/src/platform.h b/src/platform.h
index 527de1606c4453c41a5f2e8cb877b073ae510d91..8e524aeaf06f356c9a8f79de2daf6aedee023a9f 100644
--- a/src/platform.h
+++ b/src/platform.h
@@ -302,6 +302,9 @@ class OS {
   // positions indicated by the members of the CpuFeature enum from globals.h
   static uint64_t CpuFeaturesImpliedByPlatform();
 
+  // The total amount of physical memory available on the current system.
+  static uint64_t TotalPhysicalMemory();
+
   // Maximum size of the virtual memory.  0 means there is no artificial
   // limit.
   static intptr_t MaxVirtualMemory();