[Support] Use zx_cache_flush on Fuchsia to flush instruction cache

[phosek]

Summary

Fuchsia doesn't use __clear_cache, instead it provide zx_cache_flush
system call. Use it to flush instruction cache.

[mcgrathr]

lgtm

llvm/trunk/lib/Support/Unix/Memory.inc

 #ifdef HAVE_SYS_MMAN_H
 #include <sys/mman.h>
 #endif
 
 #ifdef __APPLE__
 #include <mach/mach.h>
 #endif
 
+#ifdef __Fuchsia__
+#include <zircon/syscalls.h>
+#endif
+
 #if defined(__mips__)
 #  if defined(__OpenBSD__)
 #    include <mips64/sysarch.h>
 #  elif !defined(__FreeBSD__)
 #    include <sys/cachectl.h>
 #  endif
 #endif
 
-#ifdef __APPLE__
+#if defined(__APPLE__)
 extern "C" void sys_icache_invalidate(const void *Addr, size_t len);
 #else
 extern "C" void __clear_cache(void *, void*);
 #endif
 
 namespace {
 
 int getPosixProtectionFlags(unsigned Flags) {

 #if defined(__APPLE__)
 
 #  if (defined(__POWERPC__) || defined (__ppc__) || \
        defined(_POWER) || defined(_ARCH_PPC) || defined(__arm__) || \
        defined(__arm64__))
   sys_icache_invalidate(const_cast<void *>(Addr), Len);
 #  endif
 
+#elif defined(__Fuchsia__)
+
+  zx_status_t Status = zx_cache_flush(Addr, Len, ZX_CACHE_FLUSH_INSN);
+  assert(Status == ZX_OK && "cannot invalidate instruction cache");
+
 #else
 
 #  if (defined(__POWERPC__) || defined (__ppc__) || \
        defined(_POWER) || defined(_ARCH_PPC)) && defined(__GNUC__)
   const size_t LineSize = 32;
 
   const intptr_t Mask = ~(LineSize - 1);
   const intptr_t StartLine = ((intptr_t) Addr) & Mask;