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/y2019/vision/server/BUILD b/y2019/vision/server/BUILD
index 3d63e09..c20357f 100644
--- a/y2019/vision/server/BUILD
+++ b/y2019/vision/server/BUILD
@@ -9,6 +9,7 @@
     srcs = [
         "demo.ts",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
 )
 
 gen_embedded(
@@ -17,6 +18,7 @@
         include = ["www_defaults/**/*"],
         exclude = ["www/**/*"],
     ),
+    target_compatible_with = ["@platforms//os:linux"],
 )
 
 aos_downloader_dir(
@@ -26,12 +28,14 @@
         "//y2019/vision/server/www:visualizer_bundle",
     ],
     dir = "www",
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
 )
 
 cc_proto_library(
     name = "server_data_proto",
     srcs = ["server_data.proto"],
+    target_compatible_with = ["@platforms//os:linux"],
 )
 
 cc_binary(
@@ -43,6 +47,7 @@
         "//y2019/vision/server/www:files",
         "//y2019/vision/server/www:visualizer_bundle",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
     deps = [
         ":gen_embedded",