Trying to get "host_cpu" in DEPS file may fail on AIX.
Getting the "machine ID number" using "uname -m" on an AIX box, in an LPAR
environment will not generate a unique machine identifier and instead might
return a string such as "00f9445f4c00". "platform.processor()" will be used
as a fall back on AIX.

Change-Id: I7fada10059e29066f5a13d6135b01eaeaccb769d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1504554Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>