Revert "Fix arm64 build."

runtime/vm/cpu_arm64.cc

Index: runtime/vm/cpu_arm64.cc
diff --git a/runtime/vm/cpu_arm64.cc b/runtime/vm/cpu_arm64.cc
index 7cdd8ece9e40e00931abb464d3d108cd2bf764f6..881aebc42fb166c6de00eb567bb6828115c50568 100644
--- a/runtime/vm/cpu_arm64.cc
+++ b/runtime/vm/cpu_arm64.cc
@@ -15,12 +15,6 @@
 #include <unistd.h>  /* NOLINT */
 #endif
 
-#if defined(__linux__) || defined(ANDROID)
-extern "C" {
-void __clear_cache(char*, char*);
-}
-#endif
-
 namespace dart {
 
 void CPU::FlushICache(uword start, uword size) {
@@ -38,9 +32,10 @@ void CPU::FlushICache(uword start, uword size) {
   // ARM recommends using the gcc intrinsic __clear_cache on Linux and Android.
   // blogs.arm.com/software-enablement/141-caches-and-self-modifying-code/
   #if defined(__linux__) || defined(ANDROID)
+    extern void __clear_cache(char*, char*);
     char* beg = reinterpret_cast<char*>(start);
     char* end = reinterpret_cast<char*>(start + size);
-    __clear_cache(beg, end);
+    ::__clear_cache(beg, end);
   #else
     #error FlushICache only tested/supported on Linux and Android
   #endif