Set rtc_prefer_fixed_point to true on arm64 to match GYP

BUG=None
NOTRY=True

Review URL: https://codereview.webrtc.org/1681183002

Cr-Commit-Position: refs/heads/master@{#11555}

webrtc/build/webrtc.gni

@@ -71,7 +71,7 @@ declare_args() {
     rtc_enable_protobuf = false
   }
 
-  if (current_cpu == "arm") {
+  if (current_cpu == "arm" || current_cpu == "arm64") {
     rtc_prefer_fixed_point = true
   }