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/y2017/vision/BUILD b/y2017/vision/BUILD
index a71fc8a..62e2278 100644
--- a/y2017/vision/BUILD
+++ b/y2017/vision/BUILD
@@ -1,5 +1,5 @@
 load("@com_github_google_flatbuffers//:build_defs.bzl", "flatbuffer_cc_library")
-load("//tools/build_rules:gtk_dependent.bzl", "gtk_dependent_cc_binary", "gtk_dependent_cc_library")
+load("//tools/build_rules:gtk_dependent.bzl", "gtk_dependent_cc_binary")
 load("@com_google_protobuf//:protobuf.bzl", "cc_proto_library")
 
 package(default_visibility = ["//visibility:public"])
@@ -10,17 +10,20 @@
         "vision.fbs",
     ],
     gen_reflections = 1,
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
 )
 
 cc_proto_library(
     name = "vision_result",
     srcs = ["vision_result.proto"],
+    target_compatible_with = ["@platforms//os:linux"],
 )
 
 cc_proto_library(
     name = "vision_config",
     srcs = ["vision_config.proto"],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         "//aos/vision/image:camera_params",
     ],
@@ -31,6 +34,7 @@
     srcs = [
         "target_sender.cc",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":target_finder",
         ":vision_config",
@@ -56,6 +60,7 @@
     srcs = [
         "target_receiver.cc",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
     deps = [
         ":target_finder",
@@ -73,6 +78,7 @@
     name = "target_finder",
     srcs = ["target_finder.cc"],
     hdrs = ["target_finder.h"],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         "//aos/vision/blob:threshold",
         "//aos/vision/blob:transpose",
@@ -84,6 +90,7 @@
 gtk_dependent_cc_binary(
     name = "debug_viewer",
     srcs = ["debug_viewer.cc"],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":target_finder",
         "//aos/vision/blob:move_scale",