[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..b1c53879ce8f5b18413c970517e9d1e9e7d11440 100644
--- a/runtime/vm/cpu_arm64.cc
+++ b/runtime/vm/cpu_arm64.cc
@@ -34,12 +34,15 @@ 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)
+  // TODO(magenta): Use MX_CACHE_FLUSH_INSTR when implemented.
+  // mx_status result = mx_cache_flush(start, size, MX_CACHE_FLUSH_INSN);

[Roland McGrath, 2017/07/25 22:34:24]

This should build now and the Magenta runtime support will probably land within
a day, so I think you could just enable this now.

[rmacnak, 2017/07/25 22:38:15]

I can't call it now because the current implementation is a trap instead of
no-op.

+  // ASSERT(result == MX_OK);
 #else
 #error FlushICache only tested/supported on Android, Fuchsia, and Linux
 #endif