Use new target/{x64,arm64} directory from Fuchsia SDK.

The new SDK format put the kernel images under the target/{x64,arm64}
directories.

Change-Id: I36f944cf1914289ec16d8f9595323b3797652709
Reviewed-on: https://chromium-review.googlesource.com/1087492
Commit-Queue: Alain Vongsouvanh <alainv@google.com>
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#564564}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: e822f462b0ca5c542a5381addd84dbe8b7a1d420

config/fuchsia/BUILD.gn

@@ -74,11 +74,7 @@ action("blobstore_extended_fvm") {
   # limitation in the future.
   _extend_size = "1073741824"  # 1GB
 
-  if (current_cpu == "arm64") {
-    _target_dir = "//third_party/fuchsia-sdk/sdk/target/aarch64"
-  } else if (current_cpu == "x64") {
-    _target_dir = "//third_party/fuchsia-sdk/sdk/target/x86_64"
-  }
+  _target_dir = "//third_party/fuchsia-sdk/sdk/target/${current_cpu}"
 
   script = "//build/config/fuchsia/extend_fvm.py"