Prepares for arm64 cross-build.

runtime/vm/cpu_arm64.cc

Index: runtime/vm/cpu_arm64.cc
===================================================================
--- runtime/vm/cpu_arm64.cc	(revision 36692)
+++ runtime/vm/cpu_arm64.cc	(working copy)
@@ -18,10 +18,27 @@
 namespace dart {
 
 void CPU::FlushICache(uword start, uword size) {
-#if defined(USING_SIMULATOR)
-  // Nothing to do.
-#else
-  UNIMPLEMENTED();
+#if defined(HOST_ARCH_ARM64)
+  // Nothing to do. Flushing no instructions.
+  if (size == 0) {
+    return;
+  }
+
+  // ARM recommends using the gcc intrinsic __clear_cache on Linux, and the
+  // library call cacheflush from unistd.h on 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);
+  #elif defined(ANDROID)
+    // TODO(zra): Verify that this is correct for arm64 in addition to arm.
+    cacheflush(start, start + size, 0);
+  #else
+    #error FlushICache only tested/supported on Linux and Android
+  #endif
+
 #endif
 }