platform: separate platforms by architecture

A long time ago, the mosys platforms used to be separated by vendor,
which gave them a little bit of structure, and made them easy to
navigate.  But a few challenges arose: we don't want to release the
OEM name before we launch a product, and platforms started to be used
by multiple products, often by multiple vendors.  Eventually, all the
platforms started landing in platform/experimental/ or
platform/google/, the structure separating by vendors became a useless
organization tool, and we flattened it (CL:575613).

However, separating by architecture is useful:

- Makes for easy code-searches, especially when searching for
  platforms of a certain architecture.

- Makes it easy for reviewers to see which types of platforms are
  affected by a change.

- Reduces the binary size of mosys.  For example, now 32-bit ARM only
  needs 4 platforms compiled, whereas it had 7 platforms compiled
  before.  Combine this with LTO, and we'll strip out a bunch of
  unused probing functions too.

- Reduced probing time, as mosys probes platforms sequentially, and
  we'll have less platforms to probe in each binary.

BUG=chromium:1070692
TEST=compiles, "mosys -S" shows a much smaller list of platforms

Change-Id: I4f67de82c1f8f36d54dcdab18b65efff349967e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/mosys/+/2171631
Tested-by: Jack Rosenthal <jrosenth@chromium.org>
Commit-Queue: Jack Rosenthal <jrosenth@chromium.org>
Reviewed-by: Paul Fagerburg <pfagerburg@chromium.org>
Reviewed-by: C Shapiro <shapiroc@chromium.org>
165 files changed