To build the arm ninja binary, I downloaded a zip with the ninja 1.8.2
code, and ran `./configure --bootstrap` on an arm mac, with Xcode 12 beta
installed, followed by `strip ninja`. That gave me a working ninja 1.8.2
arm binary. I then copied it to my intel mac and ran

    lipo -create -output ninja-mac ninja-mac ~/Downloads/ninja

to make a fat binary.

No intended behavior change on non-arm macs.

(This means the x86_64 slice, which is the previously existing binary,
has a LC_VERSION_MIN_MACOSX version 10.6, while the arm64 slice has
LC_BUILD_VERSION minos 11.0, according to `otool -arch ax86_64 -l ninja-mac`
and `otool -arch arm64 -l ninja-mac`.)

Bug: 1103315,762354
Change-Id: I2c4bea53a01d7a6135f9cd5554e3123dba960664
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2288417
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Mark Mentovai <mark@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>