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/y2018/control_loops/superstructure/arm/BUILD b/y2018/control_loops/superstructure/arm/BUILD
index bf3c306..d05c245 100644
--- a/y2018/control_loops/superstructure/arm/BUILD
+++ b/y2018/control_loops/superstructure/arm/BUILD
@@ -6,6 +6,7 @@
     hdrs = [
         "trajectory.h",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
     deps = [
         ":dynamics",
@@ -21,6 +22,7 @@
     srcs = [
         "trajectory_test.cc",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":demo_path",
         ":dynamics",
@@ -39,6 +41,7 @@
     hdrs = [
         "dynamics.h",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
     deps = [
         "//frc971/control_loops:runge_kutta",
@@ -53,6 +56,7 @@
         "demo_path.cc",
     ],
     hdrs = ["demo_path.h"],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [":trajectory"],
 )
 
@@ -61,6 +65,7 @@
     srcs = [
         "dynamics_test.cc",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":dynamics",
         "//aos/testing:googletest",
@@ -72,7 +77,7 @@
     srcs = [
         "trajectory_plot.cc",
     ],
-    restricted_to = ["//tools:k8"],
+    target_compatible_with = ["@platforms//cpu:x86_64"],
     deps = [
         ":ekf",
         ":generated_graph",
@@ -91,6 +96,7 @@
     hdrs = [
         "ekf.h",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
     deps = [
         ":dynamics",
@@ -103,11 +109,13 @@
     name = "graph",
     srcs = ["graph.cc"],
     hdrs = ["graph.h"],
+    target_compatible_with = ["@platforms//os:linux"],
 )
 
 cc_test(
     name = "graph_test",
     srcs = ["graph_test.cc"],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":graph",
         "//aos/testing:googletest",
@@ -122,6 +130,7 @@
     hdrs = [
         "arm.h",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
     deps = [
         ":demo_path",
@@ -143,6 +152,7 @@
         "generated_graph.cc",
     ],
     cmd = "$(location //y2018/control_loops/python:graph_codegen) $(OUTS)",
+    target_compatible_with = ["@platforms//os:linux"],
     tools = [
         "//y2018/control_loops/python:graph_codegen",
     ],
@@ -157,6 +167,7 @@
     copts = [
         "-O1",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
     deps = [
         ":graph",