Map arm64 and mips64el -> x64 for mksnapshot

snapshot_toolchain.gni

 # Copyright 2015 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 16 matching lines @@
 
 # The snapshot needs to be compiled for the host, but compiled with
 # a toolchain that matches the bit-width of the target.
 
 # TODO(GYP): For now we only support 32-bit little-endian target builds from an
 # x64 Linux host. Eventually we need to support all of the host/target
 # configurations v8 runs on.
 if (host_cpu == "x64" && host_os == "linux") {
   if (target_cpu == "arm" || target_cpu == "mipsel" || target_cpu == "x86") {
     snapshot_toolchain = "//build/toolchain/linux:clang_x86"
-  } else if (target_cpu == "x64") {
+  } else if (target_cpu == "x64" || target_cpu == "arm64" || target_cpu == "mips64el") {
     snapshot_toolchain = "//build/toolchain/linux:clang_x64"
   } else {
-    assert(false, "Need environment for this arch")
+    assert(false, "Need environment for this arch: $target_cpu")
   }
 } else {
   snapshot_toolchain = default_toolchain
 }