[fuchsia, arm64] Use mx_cache_flush instead of removed __clear_cache.

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 0c089aeff0960e05640e42fb8f46da06fb9f20a4..14142ecfd4b65eb2ec362c76ce963fb784793236 100644
--- a/runtime/vm/cpu_arm64.cc
+++ b/runtime/vm/cpu_arm64.cc
@@ -13,9 +13,11 @@
 
 #if !defined(USING_SIMULATOR)
 #if !defined(HOST_OS_FUCHSIA)
-#include <sys/syscall.h> /* NOLINT */
+#include <sys/syscall.h>
+#else
+#include <magenta/syscalls.h>
 #endif
-#include <unistd.h> /* NOLINT */
+#include <unistd.h>
 #endif
 
 namespace dart {
@@ -34,12 +36,15 @@ 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(HOST_OS_ANDROID) || defined(HOST_OS_FUCHSIA) ||                    \
-    defined(HOST_OS_LINUX)
+#if defined(HOST_OS_ANDROID) || defined(HOST_OS_LINUX)
   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(HOST_OS_FUCHSIA)
+  mx_status_t result = mx_cache_flush(reinterpret_cast<const void*>(start),
+                                      size, MX_CACHE_FLUSH_INSN);
+  ASSERT(result == MX_OK);
 #else
 #error FlushICache only tested/supported on Android, Fuchsia, and Linux
 #endif