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/input/BUILD b/aos/input/BUILD
index 24e6381..fafe39d 100644
--- a/aos/input/BUILD
+++ b/aos/input/BUILD
@@ -8,6 +8,7 @@
     hdrs = [
         "joystick_input.h",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         "//aos/events:event_loop",
         "//aos/input:driver_station_data",
@@ -24,6 +25,7 @@
     hdrs = [
         "drivetrain_input.h",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         "//aos:math",
         "//aos/input:driver_station_data",
@@ -44,6 +46,7 @@
     hdrs = [
         "driver_station_data.h",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         "//aos/robot_state:joystick_state_fbs",
         "@com_github_google_glog//:glog",
@@ -54,6 +57,7 @@
     name = "action_joystick_input",
     srcs = ["action_joystick_input.cc"],
     hdrs = ["action_joystick_input.h"],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":drivetrain_input",
         "//aos:init",