[fuchsia] Add x64/arm64 to build instructions.

Change-Id: Ie3e45813d92bab7fba0a431a4c6110c35d2931c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2427587
Commit-Queue: Sharon Yang <yangsharon@chromium.org>
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810616}

docs/fuchsia_build_instructions.md

@@ -160,6 +160,9 @@ have any Debug build-bots, it may be more broken than Release.
 
 `use_goma=true` is fine to use also if you're a Googler.
 
+Architecture options are x64 (default) and arm64. This can be set with
+`target_cpu=arm64`.
+
 ## Build
 
 All targets included in the GN build should build successfully. You can also