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/starter/BUILD b/aos/starter/BUILD
index 401d421..52fd62a 100644
--- a/aos/starter/BUILD
+++ b/aos/starter/BUILD
@@ -15,6 +15,7 @@
     srcs = [
         "starter.cc",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         "//aos:init",
         "//aos:unique_malloc_ptr",
@@ -33,6 +34,7 @@
     name = "starterd_lib",
     srcs = ["starterd_lib.cc"],
     hdrs = ["starterd_lib.h"],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":starter_fbs",
         ":starter_rpc_fbs",
@@ -51,6 +53,7 @@
         "//aos/events:pingpong_config",
         "//aos/events:pong",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":starter_rpc_lib",
         ":starterd_lib",
@@ -64,6 +67,7 @@
 cc_binary(
     name = "starterd",
     srcs = ["starterd.cc"],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":starterd_lib",
         "//aos:init",
@@ -74,6 +78,7 @@
     name = "starter_rpc_lib",
     srcs = ["starter_rpc_lib.cc"],
     hdrs = ["starter_rpc_lib.h"],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":starter_fbs",
         ":starter_rpc_fbs",
@@ -86,6 +91,7 @@
 cc_binary(
     name = "starter_cmd",
     srcs = ["starter_cmd.cc"],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":starter_rpc_lib",
         "@com_github_google_glog//:glog",
@@ -96,6 +102,7 @@
     name = "starter_fbs",
     srcs = ["starter.fbs"],
     gen_reflections = True,
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
 )
 
@@ -103,5 +110,6 @@
     name = "starter_rpc_fbs",
     srcs = ["starter_rpc.fbs"],
     gen_reflections = True,
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
 )