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/shooter/BUILD b/y2020/control_loops/superstructure/shooter/BUILD
index fe8a29a..0b65037 100644
--- a/y2020/control_loops/superstructure/shooter/BUILD
+++ b/y2020/control_loops/superstructure/shooter/BUILD
@@ -1,9 +1,8 @@
 package(default_visibility = ["//visibility:public"])
 
-load("@com_github_google_flatbuffers//:build_defs.bzl", "flatbuffer_cc_library")
-
 cc_library(
     name = "shooter_plants",
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         "//frc971/control_loops:state_feedback_loop",
     ],
@@ -17,6 +16,7 @@
     hdrs = [
         "shooter.h",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":flywheel_controller",
         "//aos/controls:control_loop",
@@ -36,6 +36,7 @@
     hdrs = [
         "flywheel_controller.h",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         "//aos/controls:control_loop",
         "//frc971/control_loops:profiled_subsystem",