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/drivetrain/BUILD b/y2020/control_loops/drivetrain/BUILD
index 7e2ba18..490b1e7 100644
--- a/y2020/control_loops/drivetrain/BUILD
+++ b/y2020/control_loops/drivetrain/BUILD
@@ -11,6 +11,7 @@
         "kalman_drivetrain_motor_plant.cc",
     ],
     cmd = "$(location //y2020/control_loops/python:drivetrain) $(OUTS)",
+    target_compatible_with = ["@platforms//os:linux"],
     tools = [
         "//y2020/control_loops/python:drivetrain",
     ],
@@ -27,6 +28,7 @@
         "hybrid_velocity_drivetrain.cc",
     ],
     cmd = "$(location //y2020/control_loops/python:polydrivetrain) $(OUTS)",
+    target_compatible_with = ["@platforms//os:linux"],
     tools = [
         "//y2020/control_loops/python:polydrivetrain",
     ],
@@ -46,6 +48,7 @@
         "kalman_drivetrain_motor_plant.h",
         "polydrivetrain_dog_motor_plant.h",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
     deps = [
         "//frc971/control_loops:hybrid_state_feedback_loop",
@@ -57,6 +60,7 @@
     name = "localizer",
     srcs = ["localizer.cc"],
     hdrs = ["localizer.h"],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         "//aos/containers:ring_buffer",
         "//aos/network:message_bridge_server_fbs",
@@ -77,6 +81,7 @@
     hdrs = [
         "drivetrain_base.h",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
     deps = [
         ":polydrivetrain_plants",
@@ -90,6 +95,7 @@
     srcs = [
         "drivetrain_main.cc",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
     deps = [
         ":drivetrain_base",
@@ -103,6 +109,7 @@
 aos_config(
     name = "simulation_config",
     src = "drivetrain_simulation_config.json",
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
     deps = [
         "//frc971/control_loops/drivetrain:simulation_channels",
@@ -113,6 +120,7 @@
 aos_config(
     name = "replay_config",
     src = "drivetrain_replay_config.json",
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
     deps = [
         "//y2020:config",
@@ -123,6 +131,7 @@
     name = "localizer_test",
     srcs = ["localizer_test.cc"],
     data = [":simulation_config"],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":drivetrain_base",
         ":localizer",
@@ -143,6 +152,7 @@
         ":replay_config",
         "@drivetrain_replay//file:spinning_wheels_while_still.bfbs",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":drivetrain_base",
         "//aos:configuration",
@@ -161,6 +171,7 @@
     name = "drivetrain_replay",
     srcs = ["drivetrain_replay.cc"],
     data = ["//y2020:config"],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":drivetrain_base",
         ":localizer",