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/control_loops/drivetrain/BUILD b/y2019/control_loops/drivetrain/BUILD
index c500ce6..78c602e 100644
--- a/y2019/control_loops/drivetrain/BUILD
+++ b/y2019/control_loops/drivetrain/BUILD
@@ -11,6 +11,7 @@
         "kalman_drivetrain_motor_plant.cc",
     ],
     cmd = "$(location //y2019/control_loops/python:drivetrain) $(OUTS)",
+    target_compatible_with = ["@platforms//os:linux"],
     tools = [
         "//y2019/control_loops/python:drivetrain",
     ],
@@ -27,6 +28,7 @@
         "hybrid_velocity_drivetrain.cc",
     ],
     cmd = "$(location //y2019/control_loops/python:polydrivetrain) $(OUTS)",
+    target_compatible_with = ["@platforms//os:linux"],
     tools = [
         "//y2019/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",
@@ -61,6 +64,7 @@
     hdrs = [
         "drivetrain_base.h",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
     deps = [
         ":polydrivetrain_plants",
@@ -74,6 +78,7 @@
     srcs = [
         "drivetrain_main.cc",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
     deps = [
         ":drivetrain_base",
@@ -88,6 +93,7 @@
     name = "target_selector_fbs",
     srcs = ["target_selector.fbs"],
     gen_reflections = 1,
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
 )
 
@@ -97,12 +103,14 @@
         "camera.fbs",
     ],
     gen_reflections = 1,
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
 )
 
 cc_library(
     name = "localizer",
     hdrs = ["localizer.h"],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         "//frc971/control_loops:pose",
         "//frc971/control_loops/drivetrain:camera",
@@ -114,6 +122,7 @@
     name = "target_selector",
     srcs = ["target_selector.cc"],
     hdrs = ["target_selector.h"],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":target_selector_fbs",
         "//frc971/control_loops:pose",
@@ -128,6 +137,7 @@
     name = "target_selector_test",
     srcs = ["target_selector_test.cc"],
     data = ["//y2019:config"],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":target_selector",
         "//aos/events:simulated_event_loop",
@@ -140,6 +150,7 @@
     name = "event_loop_localizer",
     srcs = ["event_loop_localizer.cc"],
     hdrs = ["event_loop_localizer.h"],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":camera_fbs",
         ":localizer",
@@ -161,6 +172,7 @@
         }),
     linkstatic = True,
     shard_count = 8,
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":localizer",
         ":drivetrain_base",
@@ -182,6 +194,7 @@
 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",
@@ -193,6 +206,7 @@
     name = "localized_drivetrain_test",
     srcs = ["localized_drivetrain_test.cc"],
     data = [":simulation_config"],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":camera_fbs",
         ":drivetrain_base",
@@ -211,6 +225,7 @@
     name = "drivetrain_replay",
     srcs = ["drivetrain_replay.cc"],
     data = ["//y2019:config"],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":drivetrain_base",
         ":event_loop_localizer",