[mac-arm] Mark FJCVTZS instruction as available

Mac-arm64 devices always have support for it.
Found by thakis@chromium.org

Change-Id: Id2dd9d53b66715efe8b4f7f0019d1ba06289c359
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2547281Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Auto-Submit: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71268}

src/base/cpu.cc

@@ -731,6 +731,9 @@ CPU::CPU()
     has_jscvt_ = HasListItem(features, "jscvt");
     delete[] features;
   }
+#elif V8_OS_MAC
+  // ARM64 Macs always have JSCVT.
+  has_jscvt_ = true;
 #endif  // V8_OS_WIN
 
 #elif V8_HOST_ARCH_PPC || V8_HOST_ARCH_PPC64