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The toolchain we rolled in on Wednesday fixes the underlying bugs.

1. Stop explicitly passing -mcpu=apple-a12. clang now passes this
   implicitly when handing it `-march arm64 -mmacosx-version=min=...`.
   (Note: Xcode clang also uses a `arm64-apple-macos` triple and hence
   apple-a12 with just `-march arm64`. Open-source clang currently
   defaults to `arm64-apple-ios` and hence apple-a7 with `-march arm64`.
   I asked about this at https://reviews.llvm.org/D82699#2379434 ).

2. Re-enable fatal linker warnings in arm64 component builds. The
   symbols that new clang emits no longer trigger ld64 warnings.

No intended behavior change (except fewer build warnings in arm64
component builds).

Bug: 1119249,1107955
Cq-Include-Trybots: luci.chromium.try:mac-arm64-rel
Change-Id: If6a866456461613e46ade36da436991c9d617bc5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2521213Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Mark Mentovai <mark@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824991}