[ios] Build more architecture for clang runtime library

Add "arm64" architecture to simulator version of the clang runtime
library to support `-target arm64-apple-ios*-simulator` (for Apple
Silicon cpu mac).

Bug: 1138425
Change-Id: I1f2cf794db72e9413090788ad75906837f38a983
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2490564


Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823254}