Switch everything to platforms

This patch switches the codebase over from using the "cpu"
mechanism to using bazel platforms. See
https://docs.bazel.build/versions/master/platforms.html for some more
information.

Most of the substantial changes are in //tools. Instead of using
`cc_toolchain_suite` rules, we now use regular `toolchain` rules that
are registered in the WORKSPACE. That also means that bazel now uses
the target platform to select the compiler.

All --cpu=* arguments should now be --config=* arguments. For example,
`--cpu=roborio` should now be `--config=roborio`. The CI script and
all documentation has been updated to reflect that.

The remainder of the changes revolve around tagging all targets with
`target_compatible_with`.  The old mechanism allowed us to specify
repo-wide defaults. The new mechanism does not. That means every
target that didn't have any compatibility specified, now requires
compatibility with `@platforms//os:linux`.

I used buildozer for the vast majority of `target_compatible_with`
changes. buildozer automatically buildifies any BUILD files it
touches. That means this patch also contains a few non-functional
changes that I was too lazy to remove.

Change-Id: I66d6e6ad9161520ee397597cdb492585820a3acd
diff --git a/aos/vision/tools/BUILD b/aos/vision/tools/BUILD
index 2ef3244..4be7fd7 100644
--- a/aos/vision/tools/BUILD
+++ b/aos/vision/tools/BUILD
@@ -3,6 +3,7 @@
 gtk_dependent_cc_binary(
     name = "jpeg_vision_test",
     srcs = ["jpeg_vision_test.cc"],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         "//aos/logging",
         "//aos/logging:implementations",
@@ -22,6 +23,7 @@
 cc_binary(
     name = "camera_primer",
     srcs = ["camera_primer.cc"],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         "//aos/logging",
         "//aos/logging:implementations",