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Include AEMU by default in fuchsia package runner scripts for x64 hosts.
Download AEMU package automatically for mac and linux hosts.

Bug: 1000907
Change-Id: Ia0ef798a72ba517616331d8a2dcc9b003b180546
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2026424Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Chong Gu <chonggu@google.com>
Cr-Commit-Position: refs/heads/master@{#737570}