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/turret/BUILD b/y2020/control_loops/superstructure/turret/BUILD
index 010c6fd..3761868 100644
--- a/y2020/control_loops/superstructure/turret/BUILD
+++ b/y2020/control_loops/superstructure/turret/BUILD
@@ -9,6 +9,7 @@
         "integral_turret_plant.cc",
     ],
     cmd = "$(location //y2020/control_loops/python:turret) $(OUTS)",
+    target_compatible_with = ["@platforms//os:linux"],
     tools = [
         "//y2020/control_loops/python:turret",
     ],
@@ -24,6 +25,7 @@
         "integral_turret_plant.h",
         "turret_plant.h",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
     deps = [
         "//frc971/control_loops:hybrid_state_feedback_loop",
@@ -35,6 +37,7 @@
     name = "aiming",
     srcs = ["aiming.cc"],
     hdrs = ["aiming.h"],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         "//aos:flatbuffers",
         "//frc971/control_loops:control_loops_fbs",
@@ -50,6 +53,7 @@
 cc_test(
     name = "aiming_test",
     srcs = ["aiming_test.cc"],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":aiming",
         "//aos/testing:googletest",