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/y2020/control_loops/superstructure/finisher/BUILD b/y2020/control_loops/superstructure/finisher/BUILD
index 2009886..7a628c8 100644
--- a/y2020/control_loops/superstructure/finisher/BUILD
+++ b/y2020/control_loops/superstructure/finisher/BUILD
@@ -1,7 +1,5 @@
 package(default_visibility = ["//visibility:public"])
 
-load("@com_github_google_flatbuffers//:build_defs.bzl", "flatbuffer_cc_library")
-
 genrule(
     name = "genrule_finisher",
     outs = [
@@ -11,6 +9,7 @@
         "integral_finisher_plant.cc",
     ],
     cmd = "$(location //y2020/control_loops/python:finisher) $(OUTS)",
+    target_compatible_with = ["@platforms//os:linux"],
     tools = [
         "//y2020/control_loops/python:finisher",
     ],
@@ -26,6 +25,7 @@
         "finisher_plant.h",
         "integral_finisher_plant.h",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
     deps = [
         "//frc971/control_loops:hybrid_state_feedback_loop",